Prepare 0.4 release
Add introspectables to config for registered authentication policies.
Rename register_context to register_policy.
Registering same context to multiple policies raises a configuration error.
Unregister old policy when overriding a context with another policy.
Change register_authentication_policy and authentication_policy signatures.
Removing decorator ``authentication_policy``: extension should not instantiate
authentication policy class internally.
Fix/Update README.rst, CHNAGES.rst and setup.py to be pypi compliant