Rmail to Mail: initial work

This is a work in progress to get the train moving for RMail to Mail migration, there are lots of bits around that are RMail specific which needs to be updated. This PR also includes several other fixes which should be split out and merged first:

- syck to psych
- remove iconv
- fix_encoding in util.rb (without iconv it works kind of like Decoder in heliotrope)
- using heliotrope style safe_id (MD5 of message id and refs) in stead of current message-id
- fixes to_enum errors
- use with_coder for loading/storing some sources from yaml

I have based the branch on the maildir-syncback branch with updated develop stuff merged in.

I got roughly 25k mails indexed and setup on this one, indexing is about half the speed of RMail at the time. I also started collecting problematic emails so that they can be kept for regression testing (I still need to anonymize them though)

Supports ruby 2.0, probably breaks 1.8.

implementation is based on suggestions by @rakoo in #22.

Related issues: #22, #23, #17.