Mobile platform support

[deathcap]

Modern smartphones and tablets now support WebGL, so it should be possible to get voxel.js running on them. Problems to be solved include coping with tighter resource constraints, and touch input for controls (vs mouse + hard keyboard)

Testing on an iOS device:

http://maxogden.github.io/voxel-engine/ - original three.js-based engine, actually loads and renders! Runs about 30 - 40 fps, but cannot interact (no touch input)

http://deathcap.github.io/voxel-example/ - voxel-engine#ndarray-based, repeatedly crashes with "A problem occurred with this webpage so it was reloaded." :(