initial refactoring of Threading into Archive
Merge branch 'master' into issue-110

Conflicts:
	examples/Threads.ipynb
setting index on Archive data frame to Message-ID is crucial for threading
dealing with None is the message body when cleaning the content in thread
Cleaning up Threads notebook to use new refactored code
adding 'in progress' threads notebook to save Ben's work
whitespace
Merge branch 'issue-110'. fixes #110