Add bower.json

This is the manifest file for Bower. That way, people can easily manage their local copy of multiple-select through [Bower](http://bower.io) and get new updates as they get released. :)

For your information:
- Every time you make a new release, you need to ensure that the tag name matches with the version in `bower.json`. For instance, if you create the tag 1.2.0, you need to set the version in `bower.json` to 1.2.0.
- I also registered your repository on Bower. So, when people runs `bower install multiple-select`, they fetch the latest release from your GitHub repository.