Timestamps when Cloud Publishing

[mattpodolsky]

Capture time both at the start of each sensing cycle and at the end all sensors are read, before the Publish/SD logging is done. Publish and log both or send 1 plus the elapsed time difference.

Also consider using epoch seconds instead of ISO8601 FULL format for Publishes to save message space and bandwidth. Particle's Cloud API will already supply human-readable ISO 8601 cloud timestamps that will help for debugging.