[FEATURE] Remote.prototype.requestOffer

See: https://ripple.com/build/rippled-apis/#ledger-entry

Rippled request
```js
{
  "id": 1,
  "command": "ledger_entry",
  "type": "offer",
  "offer": {"account": "rEQWVz1qN4DWw5J17s3DgXQzUuVYDSpK6M", "seq":5 },
  "ledger_index": 10850610
}
```

Response
```js
{
  "id": 7,
  "status": "success",
  "type": "response",
  "result": {
    "index": "F9DCCD26C770952D216085BF97D7E225F6A8B578ABD066AA92D7D06144B535EE",
    "ledger_index": 10850610,
    "node": {
      "Account": "rEQWVz1qN4DWw5J17s3DgXQzUuVYDSpK6M",
      "BookDirectory": "CF8D13399C6ED20BA82740CFA78E928DC8D498255249BA6351038D7EA4C68000",
      "BookNode": "0000000000000000",
      "Flags": 131072,
      "LedgerEntryType": "Offer",
      "OwnerNode": "0000000000000000",
      "PreviousTxnID": "64FAB660F9F5D9E7183E977701930AD09F372129C56E1685B68A914D8A43CA41",
      "PreviousTxnLgrSeq": 10850602,
      "Sequence": 5,
      "TakerGets": "1000000",
      "TakerPays": {
        "currency": "USD",
        "issuer": "rMwjYedjc7qqtKYVLiAccJSmCwih4LnE2q",
        "value": "100"
      },
      "index": "F9DCCD26C770952D216085BF97D7E225F6A8B578ABD066AA92D7D06144B535EE"
    },
    "validated": false
  }
}
```

src/js/ripple/remote.js

@@ -2159,6 +2159,44 @@ Remote.prototype.dirtyAccountRoot = function(account) {
   delete this.ledgers.current.account_root[account];
 };
 
+/**
+ * Get an Offer from the ledger
+ *
+ * @param {Object} options
+ *   @param {String|Number} options.ledger
+ *   @param {String} [options.account]  - Required unless using options.index
+ *   @param {Number} [options.sequence] - Required unless using options.index
+ *   @param {String} [options.index]    - Required only if options.account and options.sequence not provided
+ *
+ * @callback
+ * @param {Error} error
+ * @param {Object} message
+ *
+ * @return {Request}
+ */
+
+Remote.prototype.requestOffer = function(options, callback) {
+
+  var request = this.requestLedgerEntry('offer');
+
+  if (options.account && options.sequence) {
+    request.offerId(options.account, options.sequence);
+  } else if (options.index) {
+    request.offerIndex(options.index);
+  }
+
+  request.ledgerSelect(options.ledger);
+
+  request.once('success', function(res) {
+    request.emit('offer',res);
+  });
+
+  request.callback(callback, 'offer');
+
+  return request;
+};
+
+
 /**
  * Get an account's balance
  *

test/remote-test.js

@@ -838,5 +838,33 @@ describe('Remote', function () {
       assert.strictEqual(request.message.account_root, ADDRESS);
       assert.strictEqual(request.message.ledger_index, 'validated');
     });
+
+    it('request offer with ledger index', function() {
+      var request = remote.requestOffer({account: ADDRESS, ledger: LEDGER_INDEX, sequence: 5});
+      assert.strictEqual(request.message.command, 'ledger_entry');
+      assert.strictEqual(request.message.offer.account, ADDRESS);
+      assert.strictEqual(request.message.offer.seq, 5);
+      assert.strictEqual(request.message.ledger_index, LEDGER_INDEX);
+    });
+    it('request offer with ledger hash', function() {
+      var request = remote.requestOffer({account: ADDRESS, ledger: LEDGER_HASH, sequence: 5});
+      assert.strictEqual(request.message.command, 'ledger_entry');
+      assert.strictEqual(request.message.offer.account, ADDRESS);
+      assert.strictEqual(request.message.offer.seq, 5);
+      assert.strictEqual(request.message.ledger_hash, LEDGER_HASH);
+    });
+    it('request account balance with ledger identifier', function() {
+      var request = remote.requestOffer({account: ADDRESS, ledger: 'validated', sequence: 5});
+      assert.strictEqual(request.message.command, 'ledger_entry');
+      assert.strictEqual(request.message.offer.account, ADDRESS);
+      assert.strictEqual(request.message.offer.seq, 5);
+      assert.strictEqual(request.message.ledger_index, 'validated');
+    });
+    it('request account balance with offer index', function() {
+      var request = remote.requestOffer({ index: TRANSACTION_HASH, ledger: LEDGER_INDEX});
+      assert.strictEqual(request.message.command, 'ledger_entry');
+      assert.strictEqual(request.message.offer, TRANSACTION_HASH);
+      assert.strictEqual(request.message.ledger_index, LEDGER_INDEX);
+    });
   });
 });