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? 
I think just creating FacetButton would be a good way to start, and then we can create a more generic UI widget if we ever need it. In the future, these buttons will probably need to do even more facet-specfic things, like performing new searches when clicked, so creating a FacetButton class could give us a place to encapsulate some of that logic.