Added keyboard mode property to BPFormViewController

I was experiencing a lot of crashing trying to use BPFormViewController in a popover on iPad. It would try to move the form view, but the popover would also try to adjust, and Autolayout freaked out.

This change performs a check for a popover superview by default, if one is found the view does not try to resize in response to the keyboard. I also expose the `keyboardMode` property so you have the choice to forcibly respond to the keyboard, or not.