Add bower.json, closes #32

By defining the [main][] property, tools like [wiredep][] are able to inject
source files into `index.html` automatically (for example).

Note, Bower doesn't read `package.json`. Since this is a frontend package, you
might consider removing it.

Also, there are currently two packages of this repo in the Bower registry:

```shell
❯ bower search leaflet.fullscreen
Search results:

    leaflet-fullscreen git://github.com/brunob/leaflet.fullscreen.git
    leaflet.fullscreen git://github.com/brunob/leaflet.fullscreen.git
```

Since you're the repo owner, you might consider [unregistering][] one (or both
if you merge this pull request; re-registering afterwards) to prevent confusion.

[main]: https://github.com/bower/bower.json-spec#main
[wiredep]: https://github.com/taptapship/wiredep
[unregistering]: http://bower.io/docs/creating-packages/#unregister