Added secret field to Subscription model.
Added secret to POST data.
Added secret generation.
Added subscription check to push handling.
Added HMAC checking.
Fixed return status of pushes declined because of HMAC mismatch.
Merge branch 'feature/hmac' into develop
Removed unnecessary language setting from all tests.
Merge branch 'feature/test-localization' into develop
Merge remote-tracking branch 'origin/develop' into feature/xml-import

Conflicts:
	src/SubscriptionHandler.php
	tests/phpunit/SubscriptionHandlerTest.php