Updated Darcs backend towards supporting .children(revision).

Updated Darcs backend towards supporting .children(revision).

._vcs_isdir() and ._vcs_listdir() will need to parse the output of
  darcs show files [options] --patch REVISION PATH
but both the --patch option and the PATH argument are new, and I can't
get a recent enough version of Darcs to compile on my system.
Theoretically they will work, but they remain untested for now.

I don't think it's worth rolling my own
  darcs show files --patch REVISION
to support earlier versions of Darcs, since the only solution I can
think of now would be to check out the given revision and use
os.walk() or some such, and that would be really ugly...

Also added .version_cmp() for easy version comparison.

Reindented ._vcs_get_file_contents() to remove trailing elses since
the if clauses all contain returns.