Changing "Client" names to "Tracker" for WPStats related classes.

As @mikejohnstn pointed out, "Client" is a loaded term and "Tracker"
better jives with what these classes do so I renamed a bunch of
classes/protocols to better match this.
- WPStatsClient -> WPStatsTracker
- WPStatsMixpanelClient -> WPStatsTrackerMixpanel
- WPStatsWPComClient -> WPStatsTrackerWPCom