Implement Diceware Passphrase Generator

Note: This is a first swing. I do not have the hardware necessary to test it properly at the moment.

Fixes #163 
Probably better to make wordlist a property of a Diceware instance - e.g. assign this.wordlist = [] inside function Diceware(). This gives better encapsulation and makes testing easier.

You'll also need to update references to Diceware.wordlist elsewhere in the code so they say this.wordlist too. Keep in mind that the binding of `this' is tricky in Javascript. In particular any time you reference `this' from within a  closure, it may not refer to the same `this' as outside the closure. A common trick:

    Diceware.prototype.load = function(callback) {
      var that = this;
      $.get('...', function(data) {
         that.wordlist

Then you can be sure `that' references the Diceware object you wanted.