Improve color handling.

Inspired by @asciidisco complaining about having to write separate tests for different browsers when testing colors in his talk at feopsconf.

Different browsers have different computed style colors. This adds full CSS Color Module Level 3 support including extended color keywords.

* Replaced color utils with external library
* Special cased color comparison. Error message now output both the unchanged original and measured value, but also show the rgba syntax for comparison
* Supports a [wider range of syntaxes](https://github.com/One-com/one-color#api-overview)

The basic test cases run. I wasn't able to get firefox or chrome testing up and running in the time I had for this.