GHC URLs

Allows one to use `hsenv` like this:

    hsenv --ghc=7.6.2
    hsenv --ghc=http://www.haskell.org/ghc/dist/7.6.2/ghc-7.6.2-x86_64-apple-darwin.tar.bz2

The first form is particularly useful for setting up build environments across Linux/Windows/Mac in a uniform way.

Please let me know if there's any adjustments I can make to these patches so that they are more in keeping with the style of the project.