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

Updated winston version in the requirements to 0.7.x because (at least for me) it fails to load it if it uses the 0.5.x version.
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.
Also I added an option to add a timestamp to the meta data.