Feature/named providers

I'd like to use this bundle for an application that needs to talk to SQS queues based in different regions, so this change adds support for named providers. Essentially, you could name a provider whatever you wanted and then specify the driver (any one of aws, ironmq, or sync). To maintain backwards compatibility, providers named 'aws' or 'ironmq' are assumed to use the aws and ironmq drivers, respectively.
I flagged the wrong line; this would need to stay to allow custom validation of providers based on their driver.