Add interface for Session and add getter in server container.
Purge session from Request - and fix some styles
Still some session leftovers.
Add some docs to the sessions interface.
Merge pull request #4888 from owncloud/public_session

Public session
Merge branch 'appframework-master' into public_cache

Conflicts:
	lib/public/iservercontainer.php
	lib/server.php