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)

I'm wondering if we should just have a global `$sul-border-color` and `$sul-border-bottom-color` that could be set and be referenced. See https://github.com/sul-dlss/SearchWorks/blob/master/app/assets/stylesheets/sul-variables.css.scss#L53