Don't log message retrieve success when there are 0 messages

[hobbsh]

Hi,

First of all, thanks for the work put into pyqs! So far it seems to be a nice and simple queue processor (coming from celery...).

This is a PR to not log the 'Successfully got messages' statement if there are 0 messages. If this is unwise/unwanted, maybe we can change the loglevel to debug (logger.debug) instead? Let me know what you think. Thank you

[spulec]

I think changing to logger.debug is a good compromise. Thanks!

[hobbsh]

@spulec great, updated!

[spulec]

Thanks!