Allow matcher classes to be used directly

[Xion]

For matchers that are rarely/never parametrized (like Integer), the pair of parentheses required for their instantation is kind of a boilerplate. Ideally, we'd want the following two lines to be equivalent:

mock_foo.assert_called_with(Integer())
mock_foo.assert_called_with(Integer)

This is theoretically possible by copying the crucial magic methods (mostly __eq__) from BaseMatcher to BaseMatcherMetaclass (likely through a shared mixin). It would have to be investigated, however, that there are no adverse effects of doing so.