Implemented remaining Queue attributes. This closes spulec/moto#22

The following attributes were added:

  - ApproximateNumberOfMessagesDelayed
  - ApproximateNumberOfMessagesNotVisible
  - CreatedTimestamp
  - DelaySeconds
  - LastModifiedTimestamp
  - MaximumMessageSize
  - MessageRetentionPeriod
  - QueueArn
  - ReceiveMessageWaitTimeSeconds