A rough version for wiki diffing (#12)

Adds two radio buttons next to each wiki revision; selecting one from each column and clicking "show diff" goes to the "ugly" diff URL (?start=1&end=2) which then redirects to the "nice" diff URL (/1/2).

This version is fully working, but it lacks a few things: you can currently request reverse diffs (/2/1), and "identity" diffs (/1/1), this should be caught and handled in the WikiDiffR handler (and maybe WikiDiffProxyR too). There should also be some JS to enhance the client-side experience.

There is no reverting diffs yet, and the "quick diff" functionality hasn't been implemented either.