Documentation building client for cabal-install

Hi Hackagers, I've written a rough-and-ready documentation building service for the Hackage 2.0 effort. It is functional enough to submit build reports and build documentation for a few example packages on my local Hackage instance. Duncan: please review+commit! I also had to make some small changes to cabal-install that should go into the Cabal repo. Comments/suggestions welcome. There is one major known flaw in that the invocation of "cabal install" performed by the builder is not sandboxed. So arbitrary user code runs with the same privileges as the documentation builder itself. I guess the best solution to this is a chroot (to restrict the writeable part of the file system)+timeout? Cheers, Max