Build dependencies seperately so they don't contaminate docs.tar (fixes #130).

This patch changes the build client to run "cabal install" twice. The first pass builds any dependent packages (only) with one documentation output directory and the second pass builds the actual target package with a different documentation output directory. Only the results of the second install pass are used to create the docs.tar and so hence the documentation files pertaining to the dependencies are excluded from the docs.tar file.