refactor(modal-backdrop): make backdrop use bootstrap color

[dankahle]

Fixes #18
Looked into this issue. Was confused by the angular-strap modal docs which give you backdrop transition code, yet afford you the backdrop animation in angular-motion's am-fade code. It would be best to supply this animation for them, instead of having them have to manually put it in themselves. Maybe some people don't use angular-motion though and that's what the AS doc's transition code is all about? Tried to use the transition code here, but ran into am-fade animation code that was running in parallel, so went with animation instead. Copied the am-fade animation style with visibility and backwards fill-mode, though not sure why either one of those are there. Maybe you can clarify the need, i.e. works without visibility settings and fill-mode:none (to override am-fade's fill-mode application).

I'm not sure where all this code is used, all I know about is angular-strap. The tests are broken on a need for the karma-jasmine plugin, but didn't see any tests anyway. I tested it on angular-strap running on bootstrap 3.2.0 as the modal background still isn't fixed on bootstrap 3.3.2 last time I checked.

[dankahle]

To do this right would mean to have this code in "all" of the animations as the modal options.backdropAnimation allows it, but you haven't done that and not sure it would ever be needed. I did a modal background am-flipx the other day and it looked about as funny as you can imagine.