Add click-to-insert functionality for placeholder list

This patch turns the name of each token key into a link. When this link is clicked, the token gets inserted at the cursor point into the last textarea or textfield to receive focus.

I've created a demo video to show this functionality in action:
http://www.lullabot.com/files/TokenJSPatch.mov

Notice that the window scrolls the appropriate field into view after the token is clicked.