Added mixpanel.getPeople().identify to mark user as opted in to People tracking

Is it possible that these identify calls need to be wrapped in an if statement? I didn't dive in to the command but I was worried that
```
preferences.getString(WordPress.WPCOM_USERNAME_PREFERENCE, null);
```
might return 'null' as a value and identify non-logged in users with the same identifier.

If that's not a concern then this change should lead to profiles being pushed!