Build common REST client

Dean's Transport class implements this.

This library should be implemented to support HTTPS from the beginning.

(dtroyer):
Some ideas on organizing the library stack were outlined in an Etherpad a while back., here is a cleaned up summary of the lower layers.

* A new Session class that subclasses requests.Session and adds some additional features useful for OpenStack clients:
  * set session-wide defaults for user_agent and verify
  * encode request body to JSON as requested
  * populate headers as required: User-Agent, Forwarded, Content-Type
  * change requests' redirect handling to be more appropriate for an API
  * debug logging of request/response data
  * populate the X-Auth-Token header from an auth object contained by the Session

* An ApiVersion class that handles the mechanics of API version discovery
  * subclasses can add hacks for specific services (ie, Keystone's current 'values' struct that no other service has)

* A base Client class that contains the hooks for common operations:
  * create a new Session instance if one is not supplied
  * performs the API version discovery (see ApiVersion class below)
  * contains a ServiceCatalog instance (applications requiring multiple identity contexts at a time should use multiple Client instances)
  * define the cache interface for client-side caching of API data
  * include wrappers for the REST methods: head(), get(), post(), put(), delete(), patch()

* A base AuthPlugin class:
  * handles the specifics of authenticating a user and providing a token to Session when requested
  * creates a ServiceCatalog from the token received from Identity

* A ServiceCatalog class similar to the existing one in keystoneclient (so far)