Stabilize Import by Using Store/Storeview Codes next to IDs

[tkn98]

Like the DNS system uses names instead of numbers to make the system more robust in a distributed environment, I see great use in using store and store-view codes (which are names) instead of IDs (which are numbers) when importing data (same can be applied to exporting, too).

This change would be backwards compatible because it can be clearly differentiated between numeric ID and alpha-numeric store code (if you follow the rule to not start it with a number or you don't only use numbers).

I plan to file a PR as I got positive feedback for this feature already from my peers at work. I'm pretty sure it is much appreciated.