Update syntax for ids property in DataBehavior to include arrays

[mandragorn]

For properties on objects in an array you have to do the full boiler plate:

      updateEvents: ['behaviors.bill.data:change:receiptMessageRetrievers', 'behaviors.bill.data:fetched:ids'],
      ids: function() {
        var billDataBehavior = this.view.getBehavior('bill');
        var billData = billDataBehavior.data;
        var receiptMessageRetrievers = billData.get('receiptMessageRetrievers');
        return _.pluck(receiptMessageRetrievers, 'id');
      }

Would be nice to do something like:

      ids: { property: 'behaviors.bill.data:receiptMessageRetrievers[].id' }

[mandragorn]

possibly drop [] and auto-detect arrays.