Creating a queue with existing name is now a no-op

This is more in line with Amazon's CreateQueue API
(http://docs.aws.amazon.com/AWSSimpleQueueService/latest/APIReference/API_CreateQueue.html).

Previously, calling create_queue with a name that already existed
would cause moto to overwrite the existing queue with a new empty one.
Merge pull request #181 from ghalib/create-existing-queue-noop

Creating a queue with existing name is now a no-op