Support both Meteor.Collection.ObjectID() and the default Meteor ID strings

Meteor seems to be using two different formats as the item's ID:

- `Meteor.Collection.ObjectID()` - this is also what MongoDB uses by default.
- Meteor's ID strings - this is what Meteor uses by default when inserting elements.

This pull request simply aims to have ngMeteor support both types, since some projects use `ObjectID()`, and others might use Meteor's default ID strings.