create an access log for the proxy

currently we just log some information to the standard logger, however the granularity should be improved, and we possibly should track more information such as which repositories it failed from. This could be done with a custom plexus logger, or perhaps a separate interface that stores information by field to be able to search and analyse it without processing the log file.

We may also want to track traditional user fields such as user agent, ip, etc like a normal access log.