#41 Skeletal start to History code
#41 HistoryState.apply() implemented
#41 Use HistoryStates in Document instead of dict
#41 Some minor cleanup before replacing doc._blockchain
#41 Tests for History, various minor fixes
#41 Replace doc._blockchain with History instance
#41 Minor doc.__init__ cleanup
#41 Improve serialization code
#41 doc.version is hash of latest event
Close #41 : Cleaned up a zombie import