Add prepareTransaction() (#898)

* Export iso8601ToRippleTime(), txFlags, convertStringToHex()

* Fix a bug that caused fees exceeding 999 to throw an error

e.g. '1,000' would not be recognized as a valid number.
To use 6 decimal places, toFixed should be used instead of toFormat.

test/fixtures/requests/prepare-payment-all-options.json

@@ -24,6 +24,5 @@
   ],
   "invoiceID": "A98FD36C17BE2B8511AD36DC335478E7E89F06262949F36EB88E2D683BBCC50A",
   "noDirectRipple": true,
-  "limitQuality": true,
-  "paths": "[[{\"account\":\"rMwjYedjc7qqtKYVLiAccJSmCwih4LnE2q\",\"currency\":\"USD\",\"issuer\":\"rMwjYedjc7qqtKYVLiAccJSmCwih4LnE2q\",\"type\":49,\"type_hex\":\"0000000000000031\"},{\"currency\":\"LTC\",\"issuer\":\"rfYv1TXnwgDDK4WQNbFALykYuEBnrR4pDX\",\"type\":48,\"type_hex\":\"0000000000000030\"},{\"account\":\"rfYv1TXnwgDDK4WQNbFALykYuEBnrR4pDX\",\"currency\":\"LTC\",\"issuer\":\"rfYv1TXnwgDDK4WQNbFALykYuEBnrR4pDX\",\"type\":49,\"type_hex\":\"0000000000000031\"}]]"
+  "limitQuality": true
 }

test/fixtures/requests/prepare-payment-min.json

@@ -3,8 +3,7 @@
     "address": "r9cZA1mLK5R5Am25ArfXFmqgNwjZgnfk59",
     "amount": {
       "value": "0.01",
-      "currency": "XRP",
-      "counterparty": "rMH4UxPrbuMa1spCBR98hLLyNJp4d8p4tM"
+      "currency": "XRP"
     }
   },
   "destination": {