Rename EmailParser as EmailPreProcessor
Fix bug in EmailPreProcessor

Certain messages were causing infinite recursion, which is bad. The error
was caused by assuming that all paths through
EmailPreProcessor#strip_reply either reduced the amount of text or
returned (one of them didn't).
Add EmailParser, implement parse_email_thread
Add thread detection to EmailParser

It currently uses just the subject, and returns an EmailThread object if
one is found, or None otherwise.