Support 'key in bloom' in Python wrapper.

This is a more Pythonic usage than bloom.check(key), normally
implemented via a __contains__ method for a class written in
Python, but here via the sq_contains slot in the C code. Note
this implementation Only looks for strings (anything else
returns False).
Merge pull request #50 from peterjc/py_contains

Support 'key in bloom' in Python wrapper.