#11 adding a fallback to CanvasRenderer when WebGL is unavailable

This solves #11. The fallback can't be disabled at the moment as I would like opinions about this. 

I added `overdraw: true` line: 199 to avoid the space that appears between tiles when using CanvasRenderer (cf: https://github.com/mrdoob/three.js/issues/1044) this doesn't have effect when using WebGL.