Disable mirroring the uploaders by default

You can optionally enable it using the --mirror-uploaders option.
Let git ignore the cabal sandbox
Use the new Hackage routes by default

The option --src-is-old-hackage can be used to indicate that the source
is the old Hackage server so that the old routes are used.
Merge pull request #87 from basvandijk/new-hackage

Support new Hackage in the hackage-mirror tool