define Wallet class (#1509)

* define Wallet class
- Wallet class is a utility for deriving a wallet composed of a keypair (publicKey/privateKey).
- A wallet can be derived from either a seed, mnemnoic, or entropy (array of random numbers).
- It provides functionality to sign/verify transactions offline.

test/wallet/signTransaction/index.ts

@@ -0,0 +1,38 @@
+import {RippleAPI} from 'ripple-api'
+import {TestSuite} from '../../utils'
+import Wallet from '../../../src/Wallet'
+
+const {schemaValidator} = RippleAPI._PRIVATE
+const publicKey =
+  '030E58CDD076E798C84755590AAF6237CA8FAE821070A59F648B517A30DC6F589D'
+const privateKey =
+  '00141BA006D3363D2FB2785E8DF4E44D3A49908780CB4FB51F6D217C08C021429F'
+const address = 'rhvh5SrgBL5V8oeV9EpDuVszeJSSCEkbPc'
+
+/**
+ * Every test suite exports their tests in the default object.
+ * - Check out the "TestSuite" type for documentation on the interface.
+ * - Check out "test/api/index.ts" for more information about the test runner.
+ */
+export default <TestSuite>{
+  'sign transaction offline with txJSON': async (api) => {
+    // GIVEN a transaction
+    const txJSON = {
+      TransactionType: 'Payment',
+      Account: address,
+      Destination: 'rQ3PTWGLCbPz8ZCicV5tCX3xuymojTng5r',
+      Amount: '20000000',
+      Sequence: 1,
+      Fee: '12',
+      SigningPubKey: publicKey
+    }
+    const wallet = new Wallet(publicKey, privateKey)
+
+    // WHEN signing a transaction offline
+    const signedTx: {signedTransaction: string; id: string} =
+      wallet.signTransaction(txJSON)
+
+    // THEN we get a signedTransaction
+    schemaValidator.schemaValidate('sign', signedTx)
+  },
+}