Changeset list revamp

This pull request reworks the changeset list page so the map is fixed and the changesets scroll alongside the map.

Here's a screenshot:
![Screen Shot 2013-01-25 at 5 22 22 PM](https://f.cloud.github.com/assets/108094/99046/921c41dc-6742-11e2-9288-58784f942f60.png)


Addresses issues brought up in https://github.com/openstreetmap/openstreetmap-website/issues/203