Create asciidoctor plugin - closes #1

- Python 3.3 support
- Saner checking of plugin dependencies
- Add support for overlapping plugins
- Add priorities to plugins
- Improve documentation
- Add mock and py.test for testing
Ease to work with formats in the CLI - closes #2

- If a wrong format is introduced, the user is informed about how to list the
  available formats
- Add doconv command for listing formats
prepare for release 0.1.3