Add rest api based on django-rest-framework

Supports standard rest operations on instance, service, and all
status check models.  Creation is disabled for the StatusCheck
parent class - status checks must be created through a derived
class.

State (non-configuration) fields are omitted since they are
difficult to serialize and would be redundant (Cabot itself
is a monitoring framework).
Syntax here is wonky just FYI @andrewbaxter (sure you know that and just a typo) - have fixed