Feat : Transaction.isSimpleTransaction (#114)

* Feat : Transaction.isSimpleTransaction + tests

* Doc : Added Transaction.SimpleTransaction to doc

* Doc : Apply requested change

* chore(package): Bump version to 0.17.0

* bump patch version

* update package lock

Author: Alex-Werner

docs/transaction.md

@@ -175,5 +175,11 @@ console.log(transaction.getLockTime());
 // output similar to: Sun Nov 30 2025 00:00:00 GMT-0300 (ART)
 ```
 
+## Simple Transaction
+
+Since v0.13, simple transactions (having no more than 4 inputs), are entitled to an auto Instant Send upgrade (if mempool size is less than 10% full). 
+
+A method `isSimpleTransaction()` will simplify that verification for you.  
+
 ## Upcoming changes
 We're debating an API for Merge Avoidance, CoinJoin, Smart contracts, CoinSwap, and Stealth Addresses. We're expecting to have all of them by some time in 2015. Payment channel creation is available in the [dashcore-channel](https://github.com/dashevo/dashcore-channel) module.

lib/transaction/transaction.js

@@ -64,6 +64,7 @@ function Transaction(serialized) {
 
 var CURRENT_VERSION = 3;
 var SPECIAL_TRANSACTION_ACTIVATION_VERSION = 3;
+var SIMPLE_TRANSACTIONS_MAX_INPUTS = 4;
 var DEFAULT_NLOCKTIME = 0;
 var MAX_BLOCK_SIZE = 1000000;
 
@@ -1301,6 +1302,15 @@ Transaction.prototype.enableRBF = function() {
   return this;
 };
 
+/**
+ * Returns true if this transaction is qualified to be a simple transaction to the network (<= 4 inputs).
+ * @returns {boolean}
+ */
+Transaction.prototype.isSimpleTransaction = function() {
+  return this.inputs.length <= SIMPLE_TRANSACTIONS_MAX_INPUTS;
+};
+
+
 /* DIP2 special transaction methods */
 
 /**