Added a GIL lock to python object creator wrapper.

Without this, we get crashes if we try to create a Python object via Object::create() from C++, when the calling Python wrapper released the GIL before entering C++. We don't encounter this situation in Cortex so I've been unable to create a test case here, but this is necessary to fix a test case in Gaffer.
Merge pull request #358 from johnhaddon/objectCreatorGILLock

Added a GIL lock to python object creator wrapper.