extensions: Allow registering actions for create + delete
extensions: Allow registering actions for create + delete

This allows an extension to add new actions for create and delete
operations to an existing collection.

Currently when extending an existing collection (API namespace), an
extension can register brand new 'actions', and extend any preexisting API
functionality, but unfortunately create and delete are special cases that
don't fall under the 'actions' classification, meaning the infrastructure
can't handle them unless extending an existing impl.

Stubbing out the create/delete methods in the original resource with
the equiv of a NotImplementedError doesn't work, since 'extend'ing
requires the original implementation to exit correctly.

Whitelist 'create' and 'delete' and handle them appropriately.

v2:
    Add myself to Authors
    Update wsgi.action docs
    Add a test case

v3:
    Actually update Authors

Change-Id: I550ae93c5e200f18644042ac81656bc76dbe8955

n0ano/gantt,n0ano/gantt