Added ProvidesLogMessageAndData interface

[akutz]

This patch introduces the ProvidesLogMessageAndData interface. This interfaces enables types to directly influence the log message and data provided to a log entry when a user simply invokes log.Info(obj). This simplified syntax will make things like error handling and other log-aware types so much easier on the end-developer and result in cleaner code.

Please see the file provides_test.go for examples.