add bower.json file

While I know it is a pain to manage yet another file during releases, the backbone entry in bower is already coming from this repo, and so it would be great to use a few of the features of the bower.json file to make it's use a bit more pleasurable. 

Most notably, include the `main` attribute to list `backbone.js` as the main file, and
the `ignore` property to not include the things being ignored in `.npmignore`.