Add request(), hasNextPage(), and requestNextPage() (#887)

* Add support for all rippled APIs, including subscriptions.
* Add support for arbitrary stream message types.
* Note that rippled APIs take amounts in drops.
* request() will be available in ripple-lib version 1.0.0+

test/fixtures/responses/ledger.json

@@ -0,0 +1,23 @@
+{
+  "ledger": {
+    "accepted": true,
+    "account_hash": "F9E9653EA76EA0AEA58AC98A8E19EDCEC8299C2940519A190674FFAED3639A1F",
+    "close_flags": 0,
+    "close_time": 577999430,
+    "close_time_human": "2018-Apr-25 19:23:50",
+    "close_time_resolution": 10,
+    "closed": true,
+    "hash": "450E5CB0A39495839DA9CD9A0FED74BD71CBB929423A907ADC00F14FC7E7F920",
+    "ledger_hash": "450E5CB0A39495839DA9CD9A0FED74BD71CBB929423A907ADC00F14FC7E7F920",
+    "ledger_index": "38217406",
+    "parent_close_time": 577999422,
+    "parent_hash": "B8B364C63EB9E13FDB89CB729FEF833089B8438CBEB8FC41744CB667209221B3",
+    "seqNum": "38217406",
+    "totalCoins": "99992286058637091",
+    "total_coins": "99992286058637091",
+    "transaction_hash": "5BDD3D2780C28FB2C91C3404BD8ED04786B764B1E18CF319888EDE2C09834726"
+  },
+  "ledger_hash": "450E5CB0A39495839DA9CD9A0FED74BD71CBB929423A907ADC00F14FC7E7F920",
+  "ledger_index": 38217406,
+  "validated": true
+}

test/fixtures/rippled/index.js

@@ -37,6 +37,10 @@ module.exports = {
     usd_xrp: require('./book-offers-usd-xrp'),
     xrp_usd: require('./book-offers-xrp-usd')
   },
+  ledger_data: {
+    first_page: require('./ledger-data-first-page'),
+    last_page: require('./ledger-data-last-page')
+  },
   ledger_entry: {
     error: require('./ledger-entry-error')
   },

test/fixtures/rippled/ledger-data-first-page.json

@@ -0,0 +1,40 @@
+{
+  "id": 0,
+  "status": "success",
+  "type": "response",
+  "result": {
+    "ledger_hash":
+      "102A6E70FFB18C18E97BB56E3047B0E45EA1BCC90BFCCB8CBB0D07BF0E2AB449",
+    "ledger_index": 38202000,
+    "marker":
+      "000B714B790C3C79FEE00D17C4DEB436B375466F29679447BA64F265FD63D730",
+    "state": [
+      {
+        "Flags": 0,
+        "Indexes": [
+          "B32769DB3BE790E959A96CF37A62414479E3EB20A5AEC7156B2BF8FD816DBFF8"
+        ],
+        "LedgerEntryType": "DirectoryNode",
+        "Owner": "rwt5iiE1mRbBgNhH6spU4nKgHcE7xK9joN",
+        "RootIndex":
+          "0005C961C890079D3C4CC8317F9735D388C3CE3D9BCDC152D3C9A7C08F508D1B",
+        "index":
+          "0005C961C890079D3C4CC8317F9735D388C3CE3D9BCDC152D3C9A7C08F508D1B"
+      },
+      {
+        "Account": "rpzpyUjdWKmz7yyMvirk3abcaNvSPmDpJn",
+        "Balance": "91508000",
+        "Flags": 0,
+        "LedgerEntryType": "AccountRoot",
+        "OwnerCount": 0,
+        "PreviousTxnID":
+          "F62A5A5EC92DE4E52663B9C7B44A2B76DAB1371737C83A5A81127CBDA84DFE9E",
+        "PreviousTxnLgrSeq": 35672898,
+        "Sequence": 1,
+        "index":
+          "000B6A1287DB6174F61B1BF987E630CF41DA2A2131CFEB6C5C8143A8F539E9D1"
+      }
+    ],
+    "validated": true
+  }
+}

test/fixtures/rippled/ledger-data-last-page.json

@@ -0,0 +1,47 @@
+{
+  "id": 0,
+  "status": "success",
+  "type": "response",
+  "result": {
+    "ledger_hash":
+      "102A6E70FFB18C18E97BB56E3047B0E45EA1BCC90BFCCB8CBB0D07BF0E2AB449",
+    "ledger_index": 38202000,
+    "state": [
+      {
+        "Account": "rN3rdDNhQidDuzTFU1ArXWr89B4JG9xZ99",
+        "Balance": "249222644",
+        "Flags": 0,
+        "LedgerEntryType": "AccountRoot",
+        "OwnerCount": 0,
+        "PreviousTxnID":
+          "9A6EEBB6055E2C768BCA3B89B458A5D14A931449443053D9A1A9256F79D590DC",
+        "PreviousTxnLgrSeq": 35891744,
+        "Sequence": 1,
+        "index":
+          "000B714B790C3C79FEE00D17C4DEB436B375466F29679447BA64F265FD63D731"
+      },
+      {
+        "Account": "rLNNqGs2jJKQcg2CuoACuwkJ1ssga9LTYT",
+        "BookDirectory":
+          "6FA9AF02AF19345DC187747EF07CDABECA37CB6DCFFB045E5A08D0CF885B163B",
+        "BookNode": "0000000000000000",
+        "Flags": 0,
+        "LedgerEntryType": "Offer",
+        "OwnerNode": "0000000000000000",
+        "PreviousTxnID":
+          "5D3E557E7C08FA90EF9EE144165855B3823BD24319F28BDD81E23C3573398C44",
+        "PreviousTxnLgrSeq": 38040457,
+        "Sequence": 9,
+        "TakerGets": {
+          "currency": "CNY",
+          "issuer": "rPT74sUcTBTQhkHVD54WGncoqXEAMYbmH7",
+          "value": "322.4"
+        },
+        "TakerPays": "80000000",
+        "index":
+          "0011C33FA959278D478E7A3811D7DBB9E43E1768E12538CD54B028E5E7DA96E5"
+      }
+    ],
+    "validated": true
+  }
+}