To do
=====

* Add some more tests (test coverage *is* at 95%, btw).

* Add hg support for lasttagdiff.
  https://bugs.launchpad.net/zest.releaser/+bug/490817

* Add support for bazaar.
  https://bugs.launchpad.net/zest.releaser/+bug/490817

* Fix git tag checkout behaviour.
