Git, not Bazaar

Now that we're using Git rather than Bazaar, and Github rather than Launchpad, a bunch of things need to change in the testtools code-base.

* Documentation on how to contribute
* Launchpad as a project homepage
* The release process
* .bzrignore -> .gitignore

I've left the "series" stuff in the release documentation, although I think it's less relevant.  I have also not specified the details of how to tag. Perhaps these can be changed at around the same time as our first post-migration release?

I've also left the "upload tarball to Launchpad" logic in the release script.  Perhaps that's something to remove in a future branch.

This commit also breaks the "put revno in snapshot tarball version numbers" feature.  I don't know the right thing to do for this with git.