Use request's session instead of naked get/post requests

[hixi]

Instead of implementing our own, use the session 's get and post instead of request.get and request.post in our own get and post methods We don't need cookie persistence, but connection-pooling might be good.

This would also replace our own implementations of _data_dict() and _default_headers(): http://docs.python-requests.org/en/master/api/#requests.Session.headers

See discussion