Add skeleton of a decorator for API handler mixins

This patch adds a decorator function that generates required methods for
API handler mixins based on the given list of strings represent required
handlers.

Change-Id: I8da516c849d1216201af9e753ab9fd1fe6ce9ddb
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Modify the core plugin to load extensions

Change-Id: Iff53db4754aac857e4dd10a2be1ade975029eb14
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>