Fix relative imports so they work in both py3 and py27

synapse/push/__init__.py

synapse/push/action_generator.py

synapse/push/bulk_push_rule_evaluator.py

synapse/push/push_rule_evaluator.py

synapse/push/pusherpool.py

synapse/rest/client/v1/admin.py

synapse/rest/client/v1/initial_sync.py

synapse/rest/client/v1/login.py

synapse/rest/client/v1/register.py

synapse/rest/client/v1/room.py

synapse/rest/client/v1/voip.py

synapse/storage/__init__.py

synapse/storage/end_to_end_keys.py

synapse/storage/events.py

synapse/storage/keys.py

synapse/storage/media_repository.py

synapse/storage/signatures.py