Bug 1042960 - Create UI elements for facet bar

This is a WIP that addresses two challenges with the design for bug 1042960.

1) One-sided borders are tough. They require hacky layer lists or a 9 patch, which is doable, except:
2) How would we change their colors based on the search engine? 
The selected state indicates the currently-selected facet. In the mockup, it would be the "Web" button: https://bug1042960.bugzilla.mozilla.org/attachment.cgi?id=8461129