Added a manifest file for the bower package manager

Hi,

This adds a manifest file which will enable the [bower package manager](http://bower.io/) to introspect the immutable-js repo.

Bower's the most common solution for the packaging of front-end assets, so adding the `bower.json` file will massively reduce the overhead on integrating immutable-js into front-end projects.

If you merge the commit, you will probably want to also register the repo with the bower library, via...

`bower register immutable git://github.com/facebook/immutable-js.git`

If you don't have `bower` installed, you can install it with `npm install -g bower`.

Registering the repo will allow users to run `bower install immutable` and have the project fetched in a style akin to npm's.

Cheers,
Mark.