Add `--with-coverage` option to nosetests.
Convert Query.filter() into "catch-all" interface for handling karg searching and function filtering.
Modify `Operator` class to return new `Comparator` class so that comparison functions can be used in Query.filter().
Add `is_` and `is_not` Operators.
Proxy Operator access onto Query.__getattr__ method.
Add additional tests for new features/changes.
Update README.