Updated constructors to return instancetype to improve swift compatibility

[twobitlabs]

When using UIAlertView+Blocks in swift, the id constructors are exposed as static methods returning AnyObject!:

Updating them to return instancetype exposes some more expected initializers:

It actually feels like more of a side effect than a direct result of this change, but the instancetype methods feel cleaner and the result is more usable in Swift.