Make Curry less redundant

:fork_and_knife: 

Paired on this with @brettchalupa but another set of :eyes: would be good. The approach is to store a record that Curry left a comment in the database and associate comments with their Pull Requests. Along with the comment we store a list of the email addresses and GitHub logins that were considered unauthorized at the time of leaving the comment. Curry won't leave another comment on that Pull Request until the new comment would have a different set of unauthorized emails/logins than the previous comment.

Closes #531.