add new entry in preferences: send usage statistics
send stats opt out option prevent all analytics tracker to track any event
reword _send usage_ checkbox summary
start or end session after reloading optout option
changed clearAllData() to not endSession, and add a force option to endSession
Merge pull request #1499 from wordpress-mobile/issue/1490-mixpanel-optout-option

fix #1490 mixpanel optout option