Bump2version

- **BREAKING CHANGE** The `--bump` argument was removed, this is now the first
positional argument.
If you used `bumpversion --bump major` before, you can use
`bumpversion major` now.
If you used `bumpversion` without arguments before, you now
need to specify the part (previous default was `patch`) as in
`bumpversion patch`).

- add --no-commit, --no-tag

- If available, use git to learn about current version

- Mercurial support

- Only create a tag when it's requested (thanks gvangool)

- Initial public version