Renamed WPStats to WPAnalytics.
Renaming WPStat to WPAnalyticsStat
Renaming WPStatsTracker to WPAnalyticsTracker.
Renamed WPStatsTrackerMixpanel to WPAnalyticsTrackerMixpanel.
Renamed WPStatsTrackerWPCom to WPAnalyticsTrackerWPCom.
Renamed WPStatsTrackerMixpanelInstructionsForStat.

Renamed WPStatsTrackerMixpanelInstructionsForStat to
WPAnalyticsTrackerMixpanelInstructionsForStat
Renamed WPStat enums to correspond to new name as WPAnalyticsStat.
Updated name of Stats group to "Analytics".
Merge pull request #1568 from wordpress-mobile/issue/rename-wpstats

Rename WPStats* to WPAnalytics