GitExporter: fix username for lightweight tags

The username was previously blindly set to None, which forces Mercurial
to resort to "user@host".  Instead, get this from the Mercurial
configuration.  To support this, we read the Mercurial configuration in
GitExporter.__init__, and add util.user_config().
Add bin script for testing.

Testing gitifyhg is difficult when there's an installed egg version: eggs
are placed on sys.path ahead of anything in PYTHONPATH.  This commit adds
a bin script that puts a source-code gitifyhg on sys.path ahead of
everything else, so it can be tested.  It is NOT intended that this replace
the bin script installed by setup.py; it is only for testing.  With this
script, I can say:
PATH=/path/to/gitifyhg/bin git ...
and get the version of gitifyhg in /path/to/gitifyhg/gitifyhg.