get the filename correctly on windows

replaced the "/" with path.sep - this will allow our windows using friends nicer log messages.

observed: 

013-10-16T15:23:26.139Z <info> c:\oh\god\it\hurts\why\oh\why\lovelycode.js:35 ()  well this is great

expected:

013-10-16T15:23:26.139Z <info> lovelycode.js:35 () this is better