Insert toolbar as subview instead of just its layer

This fixes the problem in iOS 7.1 where the background of SDCAlertView
would not appear at all.

(Modified cherry pick of c3c23357a9cb0073de1fb26f1f37b9992d848650)
Merge pull request #4 from sadlerjw/fix-missing-background

Insert toolbar as subview instead of just its layer