Add IB_DESIGNABLE attribute

[Eliot Fowler]

With Xcode 6, you can now define custom UIViews as IB_DESIGNABLE and the view will actually render in Interface Builder. For example, I am creating a crossword app and need a completely custom keyboard. Using the IB_DESIGNABLE attribute, this is how my view controller renders in IB:

The crossword grid and the keyboard are both single UIViews on the view controller's view.

[Illya Busigin]

Adding this to the next release.