Fixed the (most likely never used) "@collection.link" collection

Fixed the (most likely never used) "@collection.link" collection
method, which fires off each time the collection is associated
or de-associated with a mapped object - the decorator
was not tested or functional.  The decorator method
is now named :meth:`.collection.linker` though the name "link"
remains for backwards compatibility.  Courtesy Luca Wehrstedt.
[ticket:2653]