Added bower.json file

Hiya, 

I added a `bower.json` file to your module, which allows you to register it with the [Bower package manager](http://bower.io/)

If you merge this, you still need to 

1. register it with the Bower registry:
    
    ```shell
    bower register Backbone.Undo git://github.com/osartun/Backbone.Undo.js.git
    ```
    
    (Obviously you need to have bower installed first, see the Bower website in case you need instructions)
    
1. tag the current version as v0.2.0
    
    ```shell
    git tag -a v0.2.0 -m 'release v0.2.0'
    git push origin v0.2.0
    ```

Hope you'll accept this, it would make it a lot easier to use Backbone.Undo

Fixes #1