Select specified fields on populated objects

This update adds the ability to select specific fields for populated objects using a dot notation.

The following request will add 'customer' and ''amount' to the query.select call and populate the customer field passing 'name' as a select argument to the populate call.

    GET http://localhost/api/v1/Invoices?populate=customer%select=amount,customer.name

The result should look like this:

    {amount: 10.5, customer: {name: 'Beaker'}}