Some things we can learn from pytypes

I checked the profiling hook in [pytypes](https://github.com/Stewori/pytypes/blob/master/pytypes/type_util.py#L1893), and they do two things we should also do:
- save and restore the previously active profiler
- in a thread, when the profiling callback is run after profiling has been stopped, reset the hook
(note: this should only be done after it's been stopped -- not after it's been paused)