Code review comment for lp:~michael.nelson/launchpad/635005-difference-details-2

I had to make the following changes to ensure code's use of comments renders with their own templates, rather than the default ones for IComment:

http://pastebin.ubuntu.com/496936/

As well as ensuring the zcml links up the correct template, it also ensures that the code comment classes (CodeReviewDisplayComment and CodeReviewNewRevisions) do implement the new IComment iface.