Add background and alpha support to the CSS parser

Support `background: <color>` and `background-color: <color>`. The `<color>` can now be `rgb(r, g, b)` or `rgba(r, g, b, a)`, as percentage or as a number from 0 to 255, according to the CSS3 specifications http://www.w3.org/TR/css3-color/#rgba-color and following the CSS2 grammar rules in http://www.w3.org/TR/CSS2/grammar.html

Changes the semantics of `_parseColor` so that it can return an array partially filled, to allow the missing elements to be filled from defaults (e.g. if there's no alpha specified, it's not returned) and not from its own default. Additionally, now it returns `false` to indicate parse failure. Currently, the only caller (`_parseCSS`) deals with a parse error by using the default color, so it's equivalent to returning an empty array, but the new semantics would allow for example to issue a warning, should that mechanism exist.