Added gradient handler.

This handler inserts a background gradient layer into a UIView and sets its properties. The layer is of type CAGradientLayer and therefore accepts a number of properties, most notably a "colors" array. The handler responds to the key :gradient and accepts a hash as the value. Example:

	gradient: { colors: [UIColor.redColor.CGColor, UIColor.yellowColor.CGColor] }
Merge pull request #58 from tkadauke/gradient_handler

Added gradient handler.