fix(collapse): rewrite collapse plugin to work with BS3 classes

Hi!

I managed to get the collapse/accordion functionality working properly a while back, but haven't been able to get the tests passing.

I believe the reason is that BS3 uses a class based approach, whereas the current tests rely on inline styling. So of course I looked to see about loading the bootstrap sheet for Karma tests, but it is not supported for the version ui-bootstrap uses. From what I can tell Karma added support for CSS in 0.9.5, but upgrading to the latest version involved redoing the karma.conf file, which seemed like a whole other can of worms to me :smiley: 

So, feel free to use this as a basis for the collapse rewrite. It could probably use some refactoring. Otherwise if you have some pointers, I'd be happy to assist.

Brief synopsis of what the new classes are doing:

 `.collapse` represents `display: none`,

 `.in` represents `display: block; height: auto`, and `.collapsing` takes care of removing `height: auto` for transitions.