Throw an error when an unknown order type is passed.
Renamed some utility methods.

Utils.uniquenesses_as_session_identifier => Utils.uniquenesses_as_uid
Utils.object_as_session_identifier => Utils.object_as_uid
Refactored Tracker#hit_count and Tracker#hit_count_for_session_id.

All functionality is now available in Tracker#count.
Updated changelog with new stuff.
Updated Readme.