Document building cabal from git sources and running the tests.
Merge pull request #2140 from erikd/master

Document building cabal from git sources and running the tests.
Whitespace.
Improve 'BuildingFromGit.md' a little bit.
Merge 'BuildingFromGit.md' and 'HACKING.md'.
bootstrap.sh: alter check for tarballs already downloaded to work with symlinks also
support bootstrapping cabal-install into a sandbox
Merge pull request #2137 from JakeWheat/bootstrap-sandbox

'support sandboxes' in bootstrap