Fixes #284

This appears to fix the problem that occurs when attempting to add a
mask to a displayobject that is added to a stage with a transparent
background.
Fix #288, handle black colors correctly
Merge pull request #286 from mdoch/dev

Fixes #284, allow colorMask to draw transparency
Small documentation fixes from PR #276
Merge branch 'dev' of github.com:GoodBoyDigital/pixi.js into dev
Fixed issue where stage object was not calling interactive functions
Merged from upstream.