Homepage should point to new repository

[bekaus]

Since we switch officially to github (see Ullis mail on bv-mip), the vigra homepage should point to github and not to the mercurial repo.

[hmeine]

Just wanted to let you know that I have changed the comment of the vigra hg repo to point to the new GitHub repo.

I also considered to check in a README__this_repo_has_moved.txt to make this even more visible to people who just occasionally pull. (Together with a good log entry that appears when typing "hg tip".)