Added basic API handler for PubSubHubbub subscription callbacks.
Refactored subscription checking.
Added basic SQL table for subscriptions.
Added Subscription model and validation check for subscribe events.
Fixed API module to always use the raw format and json in case an error occurs.
Added saving the subscription's status to the DB.
Added unsubscribe support to the DB.
Added unsubscribe support.
Added some tests for Subscription model.
Refactored Subscription insert test.
Added tests for 100% class code coverage.
Added code coverage ignore.
Added push handling.
Merge  branch 'feature/subscription-callback' into develop