Push notification improvements

Features

• Additional support for rich push notifications: you can now include images, buttons, and more
• You can now track when a push notification was dismissed. Replacing existing events (backwards compatible) and adding new ones: $push_notification_received, $push_notification_tap, $push_notification_dismissed for notifications sent from Mixpanel.
• Add geolocation flag to people updates (#656)

Fixes

• Always union an existing device token. Useful if the device token was removed previously but still valid.

PS Re: Just kidding. I'm still here.