Add RGBA canvas colour and idealFrame

Re Github issue #90

Permit use of RGBA on canvas in both CANVAS and WEBGL rendering modes.

Add property _Stage.rgbaColor_ to set rgba directly.

Alter _Stage.color_ property to accept rgba values (e.g. "ff008844") as well as rgb values (e.g. "abcdef").

Alter CanvasRenderer to correctly clear canvas when necessary.

Alter GLRenderManager to create consistent transparent colours when compositing to page background.

Add _ClockManager.elapsed()_ method to return master clock elapsed time.

Add _Game.idealFrame_ property to drive smooth animations.

Rebuild docs.