[SPINE] Spine fixes + strip canvas renderer improvements.

- Fixes some bugs in spine.js with the way attachments are loaded (also proposed to the spine repo at: https://github.com/EsotericSoftware/spine-runtimes/pull/334)
- Improves the strip canvas renderer to use the world transform when calculating triangle padding.

In that note, I have created a "colored mesh" class that supports per vertex coloring in canvas mode using a slightly modified version of the three.js implementation described here: http://ricardocabello.com/blog/post/710

It is very useful to create simple vertex based gradients and lighting effects, I did not make it part of pixi.js because better results could be accomplished using shaders and the built-in canvas gradient functions. Would you like me to create a PR with my class or would you rather implement it using shaders later?

Cheers!