Added code to email if it's not currently been stored.
Moving email retrieval code to separate class.
Resetting email check on sign out.
Moved email registration code to mixpanel analytics class.

Moved email registration code to WPAnalyticsTrackerMixpanel as the
extra class was redundant and the WPAnalyticsTrackerMixpanel class
hooks into the sign in flow better to fire off the register email
check.
Merge pull request #2080 from wordpress-mobile/issue/2003-retrieve-email-for-users

Populate user's email field in Mixpanel