Allow a custom ID field to be passed to WebAPIResource.get_object.
There's a case in Review Board where a resource is overriding

WebAPIResource.get_object in order to change which ID field a particular

ID is matched against. That makes it easy to get out of sync regarding

logic and optimizations.
get_object now takes an optional id_field argument. If specified, it

will use that field for the lookup. Subclasses can then make use of

the real get_object and just pass in a different value for id_field.

Tested against an accompanying change in Review Board. All unit tests passed.