better comp function
the In-Reply-To header is now properly added
Conversation now holds a refs list and doesn't do an insertion sort anymore
sort the messages before passing them to PrToPassAnalyser and gather data like number or PR messages or number of PR conversations
minor fixes
cleanup
PrToPassAnalyser became PrConversationList

* the new class actually behaves like a list that should be fed a list of
chronologically sorted messages that will be grouped in conversations where
peer review is given