add search, browse by category to explore

i need styling/ css  help on this. (Note the styling on Explore is from a plugin of mine) 

![image](https://f.cloud.github.com/assets/2665886/2506841/a3f05cc2-b3ae-11e3-87a3-b20f26a64bb6.png)

i like the functionality. @robguthrie i just pulled the search functionality from public groups. I noticed some caching in there for logged out users. that isn't in here