Added ability to throw kwargs at the Client constructor

It would be nice to have the ability to adjust the behavior of the requests.Session object. This pull request contains a patch to census/core.py that gives that ability.

The specific use case I had in mind is described in README.rst, it would be nice to be able to set a global timeout to the client.

Disclaimer: I have not tested this patch at all, as I don't personally work with this library.