Bring RESTApi closer to ithe imminent keystoneclient.Session

Prepare to use the (soon to be) common Session from keystoneclient
* Rework RESTApi to eventually be a subclass of keystoneclient.Session

Change-Id: I68e610f8b19a3f6267a93f7bf3de54a228be68aa
Update docs for plugins and release notes

* Fill out the existing command and man page
* Add a plugins page.
* Begin the release notes for 0.3.0

Change-Id: I4527fed28a10a9d79fc8f6c1d925a4bf0d0a7a36
Merge "Update docs for plugins and release notes"
Merge "Bring RESTApi closer to ithe imminent keystoneclient.Session"