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 Does this removal mean that it's no longer possible to set the background color of the `summaryBorderBottom`? 

I think we'll need to retain a reference to the  `summaryBorderBottom` view and update the `backgroundColor` in `syncUIToTheme`.