Atlas spritesheet file loader

File loader for atlas spritesheet files (e.g. used by spine)
#228

Known issues:

Trimmed image support is missing in pixi.js, the loader contains an approach like the json/spritesheet loader
Rotated images are not supported, I did not find a pixi.js solution to rotate textures, is there any?

Old PR: https://github.com/GoodBoyDigital/pixi.js/pull/240