Adding bower.json

Howdy, here's a pull request to add a bower.json file which exposes the rails.js file directly, so instead of doing the following in your Rails 4 Sprockets file:

``` javascript
//= require jquery-ujs/src/rails
```

You can now, simply, use: 

``` javascript
//= require jquery-ujs
```

Also, this makes the package a bit trimmer by omitting files that aren't needed such as Gemfile and Rakefile, the latter of which makes precompiling assets a pain.