Add Nodequeue helper class.

[sammarks]

Possible helper functions:

• nodes($nodequeue_machine_name) - Returns a list of nodes in the nodequeue.
• nids($nodequeue_machine_name) - Returns a list of NIDS in the specified nodequeue.

Each of those functions returns a NodequeueResult object, which is an ArrayObject. The query is built, when the user calls nodes() or nids(), and then only when they request an item from the array is the query actually executed.

This allows for filtering functions like withSubqueueName($subqueue_name) to filter the results and extend the query.

[sammarks]

Make this work for entity queues instead of nodequeues.