Makes BTDropInViewController conform more gracefully to a user provided theme

As above. 

I needed finer integration with the theme property in BTDropInViewController. Also made BTDropInViewController's subviews handle more gracefully user provided themes.

Perhaps someone else will find these changes useful.

Cheers,

@ggamecrazy I'm curious: Is it necessary to synthesize the `_theme` ivar? I think this should be implicit in the `@property` declaration in `BTDropInViewController.h`.