Replacing custom "log" function with system.log that comes with durandal for notifier.js

This is a suggestion to use  a unique logger through out the system. Only replaced custom "log" in notifier.js and since durandal already has a logger built in (system.log), it would be easier to use it. When it comes to production, logging can be globally switched off and I hope durandal optimizer can remove the logging from the script when optimizing.