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.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Add Tunnel Zone resource to the extension

This patch adds the tunnel zone resource extension and the API handler
mixin to our plugin.

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