Added queryset-like functionality to 'as_of' manager method
Update docstring
Simplified `as_of` logic, handle non-id pk fields
Double check for deleted items since history_date can match
Test for multiple objects for class-based `as_of`
Fix line length
Merge pull request #106 from treyhunner/non-instance-as-of

Extend the 'as_of' method to work as a class method