Rename WPStats* to WPAnalytics

* Renamed WPStats to WPAnalytics
* Renamed WPStat to WPAnalyticsStat
* Renamed WPStatsTracker to WPAnalyticsTracker
* Renamed WPStatsTrackerMixpanel to WPAnalyticsTrackerMixpanel
* Renamed WPStatsTrackerWPCom to WPAnalyticsTrackerWPCom
* Renamed WPStatsTrackerMixpanelInstructionsForStat to WPAnalyticsTrackerMixpanelInstructionsForStat

Fixes #1567