Comment on commits

Hound now complains about long subject and body lines in accordance with
<http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html>.
Introduce CommitCommentingPolicy

As not to duplicate comments.
Fix broken spec
Fix NoMethodError: undefined method `map' for nil:NilClass
Fix bug trying to post empty comment
Fix bug: subject line can be empty