Enable email-comments

This program enables email-comments in adhocracy.
Features:
- external service
- watch functions: IMAP, Maildir, mbox
- detection of comment duplicates (multiple parsing commands for same email in a fixed time period)
- secure authentication / authorization of user via email address
- parsing of multipart messages (plaintext and html emails)
- parsing of attached images (can only be rendered if markdown's safe_mode is off currently)
- deletion of (pgp) signatures
- voting
- base frame for sending HTML-emails

- self-explaining and pre-formulated notifications (about new comments) for user; these are in plain text

- test module for parsing; enhanced test-tools: make_comment