Write and read more complex git sha info

Instead of encoding the git sha into the version string, add it to
a metadata file. This will allow us to get out of the business of
arguing with pip and setuptools about version info. In order to make
this really nice, provide a command line utility called "pbr" that has
subcommands to print out the metadata that we're now including in the
egg-info dir.

Change-Id: I35a1489c9ab637769218d1e6a26d1115a52a7051