remove isArray shim

[closes #40]

src/util.js

@@ -1,12 +1,5 @@
 var Crypto = require('./crypto-js/crypto');
 
-/**
- * Cross-browser compatibility version of Array.isArray.
- */
-exports.isArray = Array.isArray || function(o) {
-    return Object.prototype.toString.call(o) === '[object Array]';
-}
-
 /**
  * Create a byte array representing a number with the given length
  */