Change to make trackingCode and packageName required ( issue #10 )

Like I've mentioned in issue #10 , I think that `packageName` and `packageVersion` should be explicit. If the user wants to use the `package.json` info from his project, he should take care of passing that info to the Insight constructor.

Also since `trackingCode` and `packageName` are crucial for Insight to work (`packageName` is used to partially name the configstore file), I added a throw Exception is any of these params is missing.