Added basic support for SQS MessageAttributes.

I think we might be able to implement this a bit cleaner by having `parse_message_attributes` raise an exception that we catch here. Take a look at what was done with the [message visibility code](https://github.com/spulec/moto/blob/f0724d458eca15e8c5c54d6aadff8fd8f01ec799/moto/sqs/responses.py#L54-62).