automatically choose button color based on contrast

While reading the compass changelog I found the [`contrast-color`](http://beta.compass-style.org/reference/compass/utilities/color/contrast/) function which allows to select on of two colors based on contrast to a third color.

In this pull request I used this function to determine the foreground color on buttons based on the background color.

Note that themes may need to be adapted if they overwrite button mixins (many of them do).