borderColor as an animation key should prefilter to the 4 individual border*Color properties

[gnarf]

UI Ticket: http://bugs.jqueryui.com/ticket/6969

Core Ticket which adds animate prefilters: http://bugs.jquery.com/ticket/8498

[gnarf]

@mikesherov - You wanted this ticket? Feel free to assign yourself here too :)

[mikesherov]

Almost done, just figuring out the test cases.