feat(modal): Add backdropClass option, similar to windowClass option

Fixes #1179

Really, identical to windowClass, but for backdrop.

@fxck: I used a different option name because we have the use case of needing both at the same time and concatenating classes like [you suggest](https://github.com/angular-ui/bootstrap/issues/1179#issuecomment-27860326) doesn't seem like the right answer when what we really want is two different classes on two different elements.