added sign_in_count to jason generated for analytics
separated logged in from signed up events in mixpanel JS view
added mixpanel-ruby gem to track contribution confirmation through the backend
added mixpanel tracking in a separate observer (in case someone wants to remove mixpanel tracking) and added tracking events for contribution confirmation
stubbed Mixpanel::Tracker new in all specs to avoid mixpanel tracking during tests
bring current_date to ruby to avoid time zone problems generating current_date (the DB is UTC always).
fixing spec problems with stubing of mixpanel observer
fixed database function can_cancel to take into account the timezone
Merge pull request #654 from diogob/mixpanel_observer

Mixpanel observer