Mod: Add Ability To Use Blocks With AsyncImageView

This update modifies AsyncImageView users may pass blocks to
AsyncImageView methods.

The update also adds a disableCrossfade method which hides the
internal implementation from the user. Formerly AsyncImageView used an
explicit boolean to define whether crossfade was enabled. This
modification preserves the new, simple implemenation, while providing an
abstraction layer for the user.