Add manpage-specific output mode

This branch adds a whole new way to generate man pages. It's not fully complete (see commit logs before) but it is a new, optional mode, that has to be explicitly enabled. I've been testing it with PlainBox see https://code.launchpad.net/~zkrynicki/checkbox/manpages/+merge/227535 with promising results.

Some known issues (but equally broken for current generator) are positional argument names not showing up correctly and lack of support for @before, @after and @replace.