Add the skeleton for the method generator

This patch adds the skeleton for the method generator for the API
request handler mixins. It doesn't implement the concrete general
processes call python-midonetclient yet.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Modify TunnelzoneHandlerMixin to use the method generator

This patch modifies TunnelzoneHandlerMixin to use supported_methods and
genrate its methods automatically instad of defining methods manually.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Add the auto generator for the extensions

This patch adds the generator of the boilerplates for the extension
classes and madifies Tunnelzone class to use it.

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