Add browser support by UMD

Hi Guys!
             I just needed to use this library browser side so I decided to implement it inside UMD wrapper:

- Add UMD (node.js and globals for now)
- Add `bower.json` to support bower.
- Add a little test case with Mocha in the browser.

Let me know what things would you change/improve.

I think this is really needed!


Thanks!
Fran