First pass at a git(hub)-enabled bootstrap.pl.  Comments welcome =)

http://bugs.dwscoalition.org/show_bug.cgi?id=4650

Requires $LJHOME to be set; accepts user's github username either
on the command line or in envvar $GITHUB_USER.  Checks out the
user's forks of dw-free and dw-nonfree to their proper locations,
then configures dw's corresponding github repos as upstream.

For gory development details, this branch is a rebase of bug4650-gitify-bootstrap.pl onto develop.