Added darcs support.

Added darcs support.

I don't know much darcs, so I make no claims about the beauty of my
implementation.  It seems to get the job done though, until a darcs
guru comes along.

I also tweaked the libbe.git.Git._rcs_get_user_id to handle the case
where user.name or user.email are not set.

I also added the option to pass a stdin string into the
libbe.rcs.RCS._u_invoke* functions.