The heartbeat feature

[mrfatguy]

Please, implement a heartbeat which will inform the server (in a separate 4th JSON, irrespectively of #16 or #11?) that the device:

• is turned off,
• is charged enough,
• has access to the Internet.

This feature is to implement "last seen at" feature on server / API side.

The heartbeat JSON / data should be sent to server every second, irrespective of other settings (#11 and #16).

It can contain no / empty payload or it can contain just a single boolean value -- whatever is needed on client side. Server side / API will ignore payload at all and will set "last seen date" in the database based on JSON's receive data.