Gold partners: add categories, support filtering

Adds categories to the gold partners, and a filtering bar at the top of the partners page:

![partner-categories](https://cloud.githubusercontent.com/assets/9033/3557045/2609ffca-092b-11e4-80df-c27aad59db65.png)

:eyeglasses: @bsimpson63 @umbrae @powerlanguage
nit: This is overspecified and will cause the selector to run the tiniest bit slower. You can just do `#category-nav a`, then it won't need to test that the tag is also a `section`.