Allow use of external collections

I need to use a collection that has been defined elsewhere in my project. I'm also using minimongoid, so that means I can't hand over creation of the collection to this module.

This commit allows you to pass either the name of the collection (original functionality) or a Collection object that you've previously instantiated.

eg:

MyCollection = new Meteor.Collection('items', {idGeneration: 'MONGO'});
Pages = Meteor.Paginate(MyCollection, {perPage: 10});