Add IBOutlet to STPView delegate property

This allows setting of the delegate in Interface Builder, which
I think is a pretty common pattern for UIView delegate properties.

I also removed the IBOutlet qualifier from the paymentView property,
since it looks like it is not intended to be set in Interface Builder.
Merge branch 'stpview-delegate-iboutlet' of github.com:kilink/stripe-ios into kilink-stpview-delegate-iboutlet
Slight tweak - keep the old IBOutlet around; make everything nonatomic.