rakefile: arbitrary directory structure under man/

https://github.com/sunaku/md2man/pull/3#issuecomment-9429077

Thanks to Postmodern for raising this issue.
hook into 'build' task only if using Bundler tasks

https://github.com/sunaku/md2man/pull/7#issuecomment-9467621

Thanks to Postmodern for raising this issue.
GH-8: Redcarpet requires Ruby 1.9 and so must we

https://github.com/sunaku/md2man/issues/8#issuecomment-9509240

Thanks to Postmodern for raising this issue.
Version 1.4.1 (2013-02-23)