Add 4G support

[jkaczman]

Adding support for optional backup cellular service in case of WiFi outage would allow for motion notifications / thumbnails / videos to still be sent over. Pairs well with #23. Could indicate in the app if it's on cellular mode or not.

Make DIY guide on how to do this.

[jkaczman]

I think it makes more sense to make this separate from the Raspberry camera device, perhaps as a base station used for all household Secluso devices. It could act as a relay when cell service is needed. Would save money since if we have five cameras, we only need 4G modem / SIM. We could utilize an ultra low power embedded device to save on backup battery since there wouldn't be much processing involved.