Fix container logs

The method `Docker::Container#logs` returns a stream from `docker`, but does so in the multiplexed fashion that we already parse from attach.  This update will fix `#logs` to use a messages object rather than concatenate onto a single string.

Since these logs are more explicitly about following the output, I updated the `Docker::Messages` object to include a list of all messages that will be returned `join`ed together after the method returns to preserve message order.

@nahiluhmot @bfulton