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.
At the next major version bump, this declaration can be removed and the next line can switch directly on `$config['providers'][$provider]['driver']`.