PYTHON-310: Added find() method to gridfs

Allows iteration through arbitrary queries against the files collection using
a new GridOutCursor class.
PYTHON-621: Fixed GridOut.read(0) returning string in Python 3
Add _retrieved param to Cursor() for advanced use cases.
Merge pull request #221 from 3rf/PYTHON-310

PYTHON-310: Added find() method to gridfs
Merge pull request #222 from 3rf/PYTHON-621

PYTHON-621: Fixed GridOut.read(0) returning string in Python 3
Work around SERVER-12331 in test_insert_manipulate_false.