Python Driver uses list.pop() unnecessarily

The Python driver calls list.pop() to consume data returned within a Cursor. This modifies the Python list, which is unnecessary. Attached is a version of Cursor.py that uses a pointer to keep track of the next element returned obviating list modification. This speeds up iteration over larger results by up to 20% in our applications. I have run the tests that ship with the Python driver with this modified Cursor successfully.