Add support for background color to the CSS parser
Change the semantics of _parseColor to allow partial arrays

This change allows _parseColor to use the defaults set in _parseCSS instead of providing its own default. It also allows to only change RGB or only change A without overriding the defaults.

Additionally, now it returns false in case of syntax error. _parseCSS reacts to that by not altering the default, so in a way returning false is equivalent to returning array().
Check for existence of the key the orthodox way
Add support for specifying CSS colors as rgb() or rgba()

This effectively adds input support for transparency in CSS format. Pull Request #5 adds input support for styles specified as arrays instead of CSS.