Allow subclasses of Idl to define a notification hook

Allow subclasses of Idl to define a notification hook

It is useful to make the notification events that Idl processes
accessible to users of the library. This will make it possible to
keep external systems in sync, but does not impose any particular
notification pattern.

The Row.from_json() call is added to be able to convert the 'old'
JSON response on an update to a Row object to make it easy for
users of notify() to see what changed, though this usage of Row
is quite different than Idl's typical use.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>