return Py_None if no user data found, to pass the test.
enable m_deterministicOverlappingPairs by default.
if m_deterministicOverlappingPairs, sort the collision pairs.
Adds unit test for the UserData functons.
Changes the char pointer in btHashString to std::string. There were
problems where the object owning the string memory would deallocate the
string, making the btHashString object invalid.
