Have context manager return mock.

moto/core/models.py

@@ -52,6 +52,7 @@ class BaseMockAWS(object):
 
     def __enter__(self):
         self.start()
+        return self
 
     def __exit__(self, *args):
         self.stop()

tests/test_core/test_context_manager.py

@@ -0,0 +1,10 @@
+import sure  # noqa
+import boto3
+from moto import mock_sqs
+
+def test_reset_api():
+    with mock_sqs() as sqs_mock:
+        conn = boto3.client("sqs", region_name='us-west-1')
+        conn.create_queue(QueueName="queue1")
+
+        list(sqs_mock.backends['us-west-1'].queues.keys()).should.equal(['queue1'])