Improves shadowed table view performance using image views.
Fixes bottom shadow appearance.
Makes minor AZShadow syntactical changes.
Replace UIGraphicsContextCreateImage with a thread-safe equivalent.
Fixes positioning of radial AZGradient.
Merge branch 'master' of https://github.com/zwaldowski/AZAppearanceKit
Improves performance of AZGradientView.

 - Uses rasterized gradient layer.
 - Uses simpler CATransactions for animation.