use Object.create for inheritance

I looked at your package code as inspiration for how to extend the Mongo.Collection constructor.  I also came across [this js reference page](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create) explaining how to use Object create to control how the object responds to `instanceof`.  The takeaway is I think you can simplify your code a bit.  

Also, as I hit same problems you would have with some properties needing to be added back to Mongo.Collection I think you need to be more careful supporting the deprecated Meteor.Collection.