Conforming to dustjs-helpers API

I think it's better if all the dust helpers follow a standard convention.
Conforming to what dustjs-helpers (and all the dustmotes modules - and several other modules) does is probably the best option.
As a plus, wth this pull request the code is able to work seamlessly in the browser and in node.js.

The code used to require formatdate would simply become:
````
var dust = require('dustjs -linkedin');
dust.helpers = require('dustjs-helpers').helpers; //HACK: https://github.com/linkedin/dustjs-helpers/issues/72
require('dustjs-helper-formatdate');
require('dustmotes-provide');
require('dustmotes-substr');
require('dustmotes-if');
require('dustmotes-iterate');
````

And for the browser:
````
<script src="/js/dust.js"></script>
<script src="/js/moment.js"></script>
<script src="/js/dustjs-helper-formatdate.js"></script>
````

@adambene:
thanks for the helper and just let me know if something doesn't suit you with this pull request :)