Add generator to copy JS file to app/assets

The included Javascript has hard-coded values in it which aren't simple to
override on-the-fly or during asset compilation. However, these values may need
to be changed depending on the host application's setup with Janrain--for
example, to enable Federate or use a different Devise model.

Add a generator to create a copy of the devise_capturable.js file in the
application's assets, so the developer can customize it as needed.