Add allow_pickle keyword to control pickle usage

[KennyChenBasis]

Fixes #50

I have kept the current defaults, though, again, we might want to change the defaults (if we were to change the defaults to match numpy's, a possible nice interface for patch would be to have it have allow_pickle=None which would mean to not patch Packer, Unpacker, etc., so they'll all use their defaults for allow_pickle).