Add a max_content_length parameter to DriverRemoteConnection in the Python client

I was recently trying to retrieve a subgraph (basically just the air-routes part of the air-routes data set) using Gremlin Python over Web Sockets. My query kept failing. After some investigation I discovered that the default maximum result set for Tornado is 10*1024*1024 (10 gig)

The current Python client does not offer a way to override this from an application. I would like to propose that we add a `max_content_length` parameter to the constructor for DriverRemoteConnection so that one could do something like:

connection = DriverRemoteConnection(
                 endpoint,
                 'g',
                 max_content_length=1024 ** 3)

I coded up a prototype for this and it seems to work well. I did not investigate if we need to also provide such a setting when connecting via HTTP yet.