Switched to Winston 0.7.x and reuse the object once created

I realised that winston will recreate the transport object on each log call so I just implemented a kind of refactoring to avoid this. Also I modified it to reuse the exchange object. Every time self.connection.exchange is called a new channel is opened (see https://github.com/postwait/node-amqp/issues/247#issuecomment-25678090) which can lead to channel overflow.