Router start and stop command processing shouldn't inherit directly from conversations handlers

Error log:

```
2013-08-18 13:20:33+0000 [WorkerAMQClient,client] Unhandled Error
        Traceback (most recent call last):
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1101, in gotResult
            _inlineCallbacks(r, g, deferred)
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1045, in _inlineCallbacks
            result = g.send(result)
          File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/service.py", line 283, in read_messages
            yield self.consume(message)
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1187, in unwindGenerator
            return _inlineCallbacks(None, gen, Deferred())
        --- <exception caught here> ---
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1045, in _inlineCallbacks
            result = g.send(result)
          File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/service.py", line 302, in consume
            message.content.body))
          File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/service.py", line 335, in consume_message
            return self.callback(message)
          File "/mnt/praekelt/vumi-go/go/vumitools/app_worker.py", line 142, in consume_control_command
            return cmd_method(*args, **kwargs)
          File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1178, in unwindGenerator
            gen = f(*args, **kwargs)
        exceptions.TypeError: process_command_start() got an unexpected keyword argument 'router_key'
```