Add requests.Session wrapper class

Wrapper class for requests.Session adds some common OpenStack functionality
* log all requests and responses at debug level
* json-encode request body passed in to request() as json=
* set default user_agent at Session creation; set to None to skip the header
* set default verify at Session creation

This sets the base for some additional functionality to follow:
* handle redirects properly for a REST API client
* handle a default auth token to se the X-Auth-Token header for simple use cases

Partially implements blueprint: rest-client
Change-Id: Ice3f2dae4a99fe59c7cc384a857900e3ae45a7d2
Add redirection handling to openstack.session.Session

The requests lib follows some browser patterns where it will redirect POSTs
as GETs for certain statuses which is not want we want for an API.
See: https://en.wikipedia.org/wiki/Post/Redirect/Get

Partially implements blueprint: rest-client
Change-Id: If8593df64bd5ff9bf3b41f990b3c0b2dcfdbbda4