added models for entity events
added triggers for determining when the is_active flags are changed on entities. Two basic tests pass
modified the trigger to use the clock timestamp rather than now() since now() is relative to the start of a transaction
updated migrations for entity relationship activation events. Added trigger for relationship activations and tests
added function for retrieving entity history for sub entities of various super entities
added tests for entity activation events
updated docs
updated docs
updated entity history imports
added data migration to initialize the history of entities and entity relationships
added ability to filter by entity ids when obtaining history
tested model manager methods
added usage in the docs
tested bulk update and create for entities
tested bulk create and delete for entity relationships
fixed flak8
removed strip_unicode function
updated force_unicode to be compatible with 2 and 3
added a reverse data migraiton
added more params to get_entities_at_times docs