Forgot to check in extensions to BaseCollection
Extract getLink helper in Base{Model,Collection}

This allows easier overrides by specific projects to handle cases such
as finding links in arrays instead of keys.
Support linkKey in relations

This allows cases where the "key" to be used for accessing a relation
doesn't match the "key" provided by the links.

For example, session.get('user') may be the preferred way of accessing a
relation specified in the links as "member".