Rewrite transaction parser and add unit test for getTransaction

src/api/ledger/parse/cancellation.js

@@ -0,0 +1,11 @@
+'use strict';
+const assert = require('assert');
+
+function parseOrderCancellation(tx) {
+  assert(tx.TransactionType === 'OfferCancel');
+  return {
+    orderSequence: tx.OfferSequence
+  };
+}
+
+module.exports = parseOrderCancellation;