make explore page navigable and pretty

Not quite ready for merge. Just throwing it up if anyone wants to take a peak and comment.

![screen shot 2014-07-23 at 5 51 12 pm](https://cloud.githubusercontent.com/assets/970124/3669098/7737c1ec-122d-11e4-84e6-463df7b9a7e0.png)

Todo:
- write a helper method for the discussion list (should pull 5 most recent public discussions/decisions in this group and all its subgroups)
- paginate all pages: /explore, /category, and search results
- bump the logo up a bit on group#show

Deploy notes:
- rake paperclip:refresh:missing_styles