Added support for mixing WebGL context inside a layer.

[tpetrina]

Canvas uses 2d context by default, it doesn't allow for optional
configuration. This creates conflicts with WebGL (e.g. with three.js
library).

For this to happen, an additional configuration parameter was necessary for both Kinetic.Canvas and for Kinetic.Layer. In the former case, a constant string was replaced, and in the latter case you can supply your own "canvas" (a class inheriting Kinetic.Canvas).

[ericdrowell]

After more thought, I've decided not to support none 2d contexts. It just doesn't make sense with KineticJS. I created several variants of KineticJS + Three.js, and I found that the easiest thing to do is to just overlay a KineticJS stage on top of a Three.js canvas, or vice versa. There's just absolutely no benefit to having this built into KineticJS. I'll put up a lab on html5canvastutorials.com that shows a clean way to combine KineticJS and Three.js. Thanks!

[tpetrina]

I look forward to that tutorial since I was unable to do it cleanly myself. Adding support for special layers seemed like the best idea.