add a link to edit this page in the footer

Adds a link in the footer to `Edit this page`, which links to the edit page in GitHub for whatever page the user is on.

![fork our code](https://cloud.githubusercontent.com/assets/4592/5210954/c8c94ee2-75a4-11e4-9164-98f75c7c784d.png)

I think we could come up with a more prominent and integrated place for this sort of thing, and I hope we do that in the coming redesign.

Fixes #302.