[Unity] SkeletonRenderer modification for color overbrightening

A modification to SkeletonRenderer to allow for a limited glowing/flashing effect, with the shader for doing the effect. Setting a color with a color component > 1 activates the effect.

![spine-overbright-demo](https://cloud.githubusercontent.com/assets/2573643/4275111/86ed185c-3cf9-11e4-8f95-e9ed47967039.gif)