fix bug in Util.cloneClass

There was a bug in the Util.cloneClass regex, which caused the newly created class to accept no parameters in its constructor:

    function OldClassName(arg1, arg2)  -->  function NewClassName()

I guess we never noticed this because the Agent constructor takes no arguments, and Agent is the only breeded (i.e. subclassed) agent in our models.** I modified the linktravel model to use a link breed instead of plain old links.

** Notice the confusing distinction between Agent and agent in that sentence! #47