Ensure that we can GET has_many relations

Augmented RestfulServer.php so that it's possible GET has_many relationships.

For example, ProposalPage (ID 5286) has_many ProposalEvents:

    private static $has_many = array(
        'ProposalEvents' => 'ProposalEvent'
    );

We want to be able to surface these via a restfulserver API call, e.g.:

http://localhost.cwp/api/v1/ProposalPage/5286/ProposalEvents.json