Use 'git rev-parse --show-toplevel' to find the repo's root-dir

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Try installing __file__ first, only using `versioneer.py` as a fallback

Signed-off-by: Giel van Schijndel <me@mortis.eu>
do_vcs_install(): add versioneer.py, not .pyc
hide the scary git message when running in a non-git tree

To do this, we hide stderr while running the 'git rev-parse
--show-toplevel' command that is used to find out where the root of the
git checkout is.
define our own os.path.relpath, to maintain support for python-2.5