Add sample API usage for getting balances and making a payment

docs/samples/README

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+Usage:
+babel-node balances.js
+babel-node payment.js (requires setting address and secret in source file first)

docs/samples/balances.js

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+'use strict';
+const RippleAPI = require('../../src').RippleAPI; // require('ripple-lib')
+
+const api = new RippleAPI({servers: ['wss://s1.ripple.com:443']});
+const address = 'r3kmLJN5D28dHuH8vZNUZpMC43pEHpaocV';
+
+api.connect(() => {
+  api.getBalances(address, {}, (error, balances) => {
+    console.log(JSON.stringify(balances, null, 2));
+    process.exit();
+  });
+});

docs/samples/payment.js

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+'use strict';
+const RippleAPI = require('../../src').RippleAPI; // require('ripple-lib')
+
+const address = 'ENTER ADDRESS HERE';
+const secret = 'ENTER SECRET HERE';
+
+const api = new RippleAPI({servers: ['wss://s1.ripple.com:443']});
+const instructions = {maxLedgerVersionOffset: 5};
+
+const payment = {
+  source: {
+    address: address,
+    amount: {
+      value: '0.01',
+      currency: 'XRP'
+    }
+  },
+  destination: {
+    address: 'rKmBGxocj9Abgy25J51Mk1iqFzW9aVF9Tc',
+    amount: {
+      value: '0.01',
+      currency: 'XRP'
+    }
+  }
+};
+
+api.connect(() => {
+  console.log('Connected...');
+  api.preparePayment(address, payment, instructions, (error, txJSON) => {
+    console.log('Payment transaction prepared...');
+    const signedTransaction = api.sign(txJSON, secret).signedTransaction;
+    console.log('Payment transaction signed...');
+    api.submit(signedTransaction, (submitError, response) => {
+      console.log(submitError ? submitError : response);
+      process.exit(submitError ? 1 : 0);
+    });
+  });
+});