Bump2version

- Fix --tag default (thanks keimlink)

- add {now} and {utcnow} to context
- use correct file encoding writing to config file. NOTE: If you are using
Python2 and want to use UTF-8 encoded characters in your config file, you
need to update ConfigParser like using 'pip install -U configparser'
- leave `current_version` in config even if available from vcs tags (was
confusing)
- print own version number in usage
- allow bumping parts that contain non-numerics
- various fixes regarding file encoding

- bugfix: tag_name and message in .bumpversion.cfg didn't have an effect [9](https://github.com/peritus/bumpversion/issues/9)

- add --tag-name option
- now works on Python 3.2, 3.3 and PyPy

- bugfix: Read only tags from `git describe` that look like versions

- bugfix: `--help` in git workdir raising AssertionError
- bugfix: fail earlier if one of files does not exist
- bugfix: `commit = True` / `tag = True` in .bumpversion.cfg had no effect