Stats: Refs #874. Retains the selection under Visitors and Views tab when rotated.

The selection wasn't being saved and persisted when the fragment was destroyed and re-created.
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into develop
Merge branch 'develop' of https://github.com/wordpress-mobile/WordPress-Android into develop
Stats: Refs #874. Using setRetainInstance(true) retains the selection under Visitors and Views tab, when rotated.
fix hard coded string
fix #1254 and style
Adding WPStats and WPStatsTrackerMixpanel.

WPStats will replace WPMobileStatsUtil as the new class to handle all
stats related calls in the Android app.
Adding some initial code to mixpanel tracker.
Merge pull request #1255 from wordpress-mobile/issue/1254-npe-mediaeditfragment

fix #1254 npe mediaeditfragment
Added WPStats endpoints for themes and reader.
Disabling WPMobileStatsUtil's mixpanel calls for now.
Removing WPMobileStatsUtil unused event as it's a part of WPStats.
Adding WPStats tracking for stats.
Fixing crash in WPStatsTrackerMixpanel
Added mixpanel.getPeople().identify to mark user as opted in to People tracking
Merge pull request #1259 from mixpanelsteve/develop

Added mixpanel.getPeople().identify to mark user as opted in to People tracking
Adding initial WPStats for editor related events.
Adding more WPStats for editor related events.
Adding WPStats events for notifications.
Adding WPStats for tracking the opening of various activities.