Publishing command now supports custom commit messages

Merged, fixed and polished @pimpin's work. Changes made:

- added `--message` argument to the `publish` command
- propagated the message to Apiary's publishing API
- updated & realigned the help screen

For now I only tested this "by hand". Current test suite doesn't cover most of the gem's code anyway, so I suggest larger revision & refactoring in the future, which would also include completing the suite. I'd volunteer to do that :)