Set client cookies from PiwikTracker.php

This pull request adds variables, functions, and logic so PiwikTracker can update client cookies when the VisitorId is set or updated.  This code is derived from the patch submitted (and ultimately not-accepted) in ticket #2699 (http://dev.piwik.org/trac/ticket/2699) and may suffice to meet the request in ticket #3239 (http://dev.piwik.org/trac/ticket/3239).

This is my first pull request so I apologize if anything (especially spacing or line breaks) is messed up.  Please let me know if I need to make any adjustments at my end.