mock/rest: 'HEAD /folders/id/messages' + fake messages provisioning implementation
mock/rest: List messages implementation 'GET /folders/id/messages'
mock/rest: use 'parent_id' as key for FID for messages
mock/rest: Fix pep warning that we may eventually use '' as dictionary
mock/rest: Implement common functionality for creating messages
mock/rest: Implement 'POST /calendars' interface
mock/rest: implement 'GET /calendars/(id)/' interface