feat(modal): modal now supports bootstrap3

This is an update to the `$modal` service so that it supports bootstrap3. @pkozlowski-opensource, feel free to take a look at this before the merge and see if you want to make any changes to the master `$modal` and take them from this. As it stands now `$modal` is incompatible between versions because the backdrop click has to be on modalBackdrop in 2.3 and modalWindow in 3.

Given Bootstrap 3 is now official, it may not be worth the effort to make it work with both.