Manpage

You mentioned a manpage in #145. Yes, we kind of like those in Debian, so here's one.

Unfortunately, it comes with a caveat.
ronn isn't installable on Ruby 1.9.2, which will cause Travis to be unhappy. So, either we need to not build it in the default `rake` task (and drop the dependency in `Gemfile`), or stop testing with Ruby 1.9.2 on Travis.

Your move?