Ok, I did it :). Made a patch also. (http://pastebin.com/028wDbJR)
Changelog:
I've modified *api.update_page()* to save the new diff information. I've added a *Page.Revision.Diff* class for storing the diff text and revto id. I've modified *site.loadrevisions()* method to support rvdiffto parameter.
A method from Page.py is still missing to get diffs just like you get a revision now. But you can get the diff text from page._revision[id].diff.text
Please review the ugly as hell patch that I made. Any comment is welcome !.
--Matias