Adds sys.capabilities

which supercedes cc.config.platform.

sys.capabilities enumerates the platform capabilities like:
  - accelerometer, keyboard, touches, opengl, etc...
Merge branch 'master' of https://github.com/cocos2d/cocos2d-html5
Merge pull request #724 from linshun/ricardoquesada-system_additions

Ricardoquesada system additions