Slight modifications to the way the event tracker works.

- Stopped automatically using identify/alias in favour of a manual
approach. You now need to explicitly call #mixpanel_identify or
#mixpanel_alias on sign-in/sign-up.

- Added #mixpanel_identify and #mixpanel_track_charge methods.

- Rearranged the output order for the JS methods to accurately track
changes to People profiles (call mixpanel.register before mixpanel.people.set).