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).
Simple API documentation, filtering tests.
API documentation clarification, fixes.
Merge branch 'rest_api' of github.com:andrewbaxter/cabot into andrewbaxter-rest_api
Markdown dependency syntax