Add a perf test for CounterTrack class

This patch adds a performance test for the <tt>CounterTrack</tt> class which was redesigned in #633. The results indicate that the redesign led to a ~12% performance regression:

![countertrack performance](https://cloud.githubusercontent.com/assets/2546601/5164672/59158a30-73d2-11e4-827a-b9857386af89.png)

Done.