Add find_one() method for gridfs.

add a find_one() method for gridfs, compare with the method find().
I suggest using the existing GridFS.find method, which returns a GridOutCursor. Something like this should work:
```
for doc in self.find(spec_or_id, *args, **kwargs):
    return doc

return None
```
Compared to your code, this would save a round-trip to the server. It would also reuse some logic in the GridOutCursor to prevent exhaust cursors and do other important things for GridFS.