Log Message:
0.5.1:
fix a bug and add tests for case where firstresult hooks return None results.

0.5.0:
fix bug where callbacks for historic hooks would not be called for already \ 
registered plugins.
fix 17 by considering only actual functions for hooks this removes the ability \ 
to register arbitrary callable objects which at first glance is a reasonable \ 
simplification, thanks @RonnyPfannschmidt for report and pr.
fix 19: allow registering hookspecs from instances. The P R from @tgoodlet also \ 
modernized the varnames implementation.
resolve 32: split up the test set into multiple modules.
resolve 14: add full sphinx docs.
add hook call mismatch warnings.
resolve 44: move to new-style classes.
add baseline benchmarking/speed tests using pytest-benchmark in P R 54.
update the README to showcase the API.
deprecate __multicall__ and add a faster call loop implementation.
raise a comprehensible error when a hookimpl is called with positional args.
fix the firstresult test making it more complete and remove a duplicate of that test.