Speed up maildir processing

1. Only process messages when emails are added (not when they are
changed or removed).
2. Only process the messages that were added, not the entire maildir
Call run as process_maildir doesn't exist anymore
Merge pull request #56 from kjg/maildir-speedup

Maildir speedup
Add John Cant to contributors list
Remove broken mailman guide link
Bump gem version to 0.5.4
Add Ryan Stawarz to the contributors list
imap: Search for unseen messages, mark processed messages as seen

Previously the IMAP receiver deleted messages that were processed. This is drastic in some situations, so we default to marking the messages as seen.

To restore the previous deletion behavior, set the done_flags option:

    Mailman.config.imap = {
      # ...
      done_flags: [Net::IMAP::DELETED]
    }

Closes #64
Bump gem version to 0.6.0
Fix link in changelog
Add public key for gem signing
Change homepage to github

As http://mailmanrb.com (no longer?) exists, change this to github.com.

So the link on the rubygems page should lead to this repository.
Merge pull request #65 from leifbladt/patch-1

Change homepage to github
Merge branch 'refs/heads/master' into headercondition