Add support for "rgba" to Phaser.Stage.backgroundColor

Ref. https://github.com/photonstorm/phaser/issues/1234

 - backgroundColor now uses valueToColor which supports hex strings, web/rgba strings, and hex numbers.
 - valueToColor normalizes across hexToColor/webToColor/getRGB; see comments
 - hexToColor now documents the prefix being optional and allows the `0x` prefix.
 - webToColor now exctracts an alpha channel if present - as a slight misfeature it will also accept `rgb(..,a)`.