Allow loading tools.xml from mirror *or* s.b.e

Using full http://s.b.e/snapsource/tools.xml disallows local repository.
Using relative tools.xml disallows running from local disk but getting tools from s.b.e.
So we try both.
Improve sbe-or-relative to generalize

New method GetURLsbeOrRelative(url) takes a relative URL and tries it first on s.b.e and if that fails as a relative url.
Merge pull request #509 from jmoenig/bh

Bh