User diff view with some stats to resolve #202!

Hey dudes/dudettes! I wrote the diff view and template requested by #202. Two questions still remain:
- [x] Is there a good/easy/reasonable way to improve the CSS on each of the 3 diff columns so that more than one badge is displayed per line? This is the same sort of issue we have with the index view. If not, we could center the badges in the columns, or better yet, display both the thumbnail and the badge name, since we have room.
- [x] Where are we going to inject this functionality? Do we want it on the user view, with a text box saying something like (compare this user with...)? If so, where -- the left or right column, and above/below what? Do we want it somewhere on the explore view? Something entirely different? Looking for input on this.

Probably should resolve these two points somehow before we merge in. Also accepting tweaks and other criticisms. I have only tested it with a small amount of dummy data, so I may have missed catching some exception(s) somewhere, but it *seems* to be working fine.