Include wiki as submodule and build manpages

As discussed in #265:

Set up the wiki as a submodule below dir/ and generate manpages
from the wiki using pandoc, add a task :doc which :gem is dependant
on. Manpages are included in the gem in the 'man' sub dir. It will
be up to the distro to install them.

New manpages must be specified in the Gemspec and Rakefile.