Pr/named colors

Some named colors defined in [`colors.js`](https://github.com/LearnBoost/stylus/blob/master/lib/colors.js) are wrong, according to the [W3C Spec](http://www.w3.org/TR/css3-color/#svg-color). This PR fixes them all and includes simple a test case. 

For this to work, I needed to hack a bit and translate the `tan()` bif from Stylus into a *unnamed* JS function, since there exists a color named `tan`. 

I did not want to modify the tests, so instead of simply returning `Math.tan(angle)`, I've followed the original implementation, which computed [the quotient](https://github.com/LearnBoost/stylus/blob/master/lib/functions/index.styl#L64) of the sinus and the cosinus of the input radius, both **rounded to 9 decimal places**, and then rounded again to 9 decimal places. I don't know why these functions round the result, but it looks totally unnecessary to me.