Update styling for facets (+ some global bootstrap panel override).

Closes #446 

**Note:** Added new `bootstrap-overrides` css file to handle overriding bootstrap styling that isn't governed by variables.  I understand that is not exactly the case w/ panels, however we want a granularity w/ the border styles that bootstrap doesn't appear to provide (AFAICT).  If folks don't like this approach I'm more than happy to change it if they have a good idea.

![facets-styling](https://cloud.githubusercontent.com/assets/96776/3579046/da6ded50-0bb5-11e4-9889-d4c160f94564.png)

@mejackreed added `$sul-panel-border` and `$sul-panel-border-bottom` variables.  I think we may want to have granularity between the document and panel styling (although maybe I'm thinking too much into it).  Let me know if that addresses your concerns.