Support having the zipfile not be the first thing on the path.

Now we have genfiles/third_party.zip/...

Now we *do* assume zipfiles end with .zip, and .zip-things are zipfiles.
That's probably ok.

Auditor: kamens,alpert

Test plan:
Did a non-standard deploy and visited:
   https://znd-mr-status-dot-khan-academy.appspot.com/mapreduce/status
and verified it loaded ok.