Adding ability to limit messages by max size
 Adding a new setting for maximum message size
 Adding method to limit a set of message uids by size
 Switching to IMAP uids so that we can trust that subsequent
 commands will use the same ids
Teaching people how to use the settings
fixing typo in the docs
Refactored the imap transport based on the tests
Testing the switch to uid-based IMAP calls required better testing
The better testing revealed some code that wasn't so pretty

I fixed the tests and the code
I'll get the 2to3 changes right someday
Merge remote-tracking branch 'alexlovelltroy/size_limiting' into issue_19
Minor stylistic changes.