[Mixpanel +deviceInfoProperties] should load properties that will not change during execution once, not each time it is called, for efficiency.
[Mixpanel +deviceModel] caches its result since it will never change.
[MixpanelPeople +deviceInfoProperties] caches properties that do not change during execution for efficiency.
Fix a rare crash per https://github.com/mixpanel/mixpanel-iphone/pull/54
Archive pending data if the application is terminated in the background before they are all posted.