Make identifier name configurable for AbstractRestfulController

[weierophinney]

Currently, AbstractRestfulController::getIdentifier() hard codes a lookup on an id parameter in either the route matches or the query string. This is not optimal in the case of child resources, as it forces the creation of additional routes to ensure that the :id segment is specific to the given child resource:

'parent' => array(
    'type' => 'Segment',
    'options' => array(
        'route' => '/api/parent[/:id]',
    ),
),
'child' => array(
    'type' => 'Segment',
    'options' => array(
        'route' => '/api/parent/:parent/child[/:id]',
    ),
),

versus the more semantic:

'parent' => array(
    'type' => 'Segment',
    'options' => array(
        'route' => '/api/parent[/:parent]',
    ),
    'may_terminate' => true,
    'child_routes' => array(
        'child' => array(
            'type' => 'Segment',
            'options' => array(
                'route' => '/child[/:child]',
            ),
        ),
    ),
),

This PR makes the identifier name configurable. BC is retained, as it still defaults to "id"; however, new methods allow you to inject an alternate identifier name, and that name will be used by getIdentifier() in order to look up the resource ID.

[danizord]

👍