Use batman-rails instead of vendored batman

This is an effort to replace the vendored version of batmanjs with the batman-rails gem (see #55).

The current code seems to work, but it has some issues:

- I can't confirm if every widget is still working after the upgrade from 0.14.1 to 0.16.1. Are there tests?
- I still have to see if there's more scripts that we can include from batman.
Are you sure? It's only there to notify that our engine depends on `batman-rails`' engine, so it makes more sense to me to be in the engine file. Only problem I see is that we're including everything from batman-rails, but that's only because they don't have their engine in a separate file.