Shell Monad and my zurihac2014 project

Hello all, My Zurihac2014 project was to address https://github.com/haskell/cabal/issues/975. I thought the cleanest way was to make two passes: 1. put executeInstallPlan into a Shell Monad to force printing debug/status messages through the monad, and track the current status of the parallel build. and 2. Use this information to write a nice UI I have implemented (1) and here is the pull request https://github.com/haskell/cabal/pull/1940. I'd appreciate if someone could make a thorough code review as it's my first commit to cabal and I only understand a small corner of the code base. If this is accepted then I also look forward to working on (2), probably based on ncurses. Best, Greg