Implicit animation support for `CALayer`

[jtrivedi]

This patch does a number of things

• Adds support for animating properties on CALayers that are not backing a UIView.
• Adds support for animating shadowColor, opacity, borderWidth, and borderColor on CALayer
• Creates a more readable "header" file that lists the supported animatable properties on UIView and CALayer
• Removes some (currently) unused code like relativePriority, which was added for affine rotation support
• Adds some tests for the new animatable properties