Added the ability to use a different revision of a page.

Hello, I wanted to see if this is something you guys would be interested in using before adding more to this pull request. This pull request adds a button called ```Revisions``` to each page:
![](http://i.imgur.com/yljPLKs.png)

This button leads to a page listing all the revisions for a given page, which the user can select to open in the editor:
![](http://i.imgur.com/d2mSAuK.png)