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.
new release-checks.sh test to detect translated lang on Glotpress not present locally
update update-translations.sh script to use exported-language-codes.csv
fix #1249
pull zh-tw from glotpress to zh-rTW and zh-rHK locally
Merge branch 'develop' into feature/language-pull-script
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
Adding initial WPStats for editor related events.