CVS converted to Mercurial

The original CVS repo has been converted to Mercurial. The complete history
should still be available. The old cvs repo is still available at present but
it will probably be removed fairly soon.

The tag names have been changed from the form REL-1-6-0 to v1.6.0 since the
only reason that they did not originally use the normal dotted notation was
that it was not allowed in CVS tags.