Add modem example (serialhost)

[ed7coyne]

Added in #66 but moved here when that was abandoned.

[ed7coyne]

Only remaining issue was:

@proppy
I think by default we should be using the default Serial so that it work with most board over usb (and thru the Arduino IDE).

@ed7coyne
Hmm I am not sure I agree. If we are showing an example of setting up a SerialHost I think the main serial line should be debug back to the user and we should provide the secondary line as data serial. That way they can easily the sample with other devices (rpi, arduino, etc..), which is the main purpose of the class. Although this may just be my ignorance, are there pins that also access the main serial line? I assumed it only went to usb.
Otherwise we are only demonstrating how to talk to themselves.

[proppy]

Although this may just be my ignorance, are there pins that also access the main serial line? I assumed it only went to usb.

Yes, the usbserial is the same as the regular TX/RX pins. It doesn't seems ESP has more than 1 hardware UART routed to the module (see https://github.com/esp8266/Arduino/blob/633e48f3aec5f1c3c11d4498fc90d378d49e6e9f/cores/esp8266/uart.c#L27) so if we choose other pin, we would be effectively stealing 2 other digital I/O.

Maybe we could interleave the DEBUG message with the main UART port (with a ! prefix so that we can easily discard it) and turn them on and off with a dedicated command.

[ed7coyne]

I don't think stealing 2 other I/O pins are an issue here. The host here is only acting as a gateway to the cloud so it does not need any of it's local i/o pins for anything other than serial.

Interleaving with the main serial link would work but I don't see a lot of value in it. To me the primary usecase here will be debugging interactions with another device, so you connect this modem example on an esp to an Arduino and run it. In that case you want to see the debug output on the PC while it communicates with the Arduino.

[proppy]

I think it would be nice if people could try the modem from the Arduino IDE without connecting an additional board for the first time.

It's a good way to play around with firebase from the Serial Monitor, maybe we could make that a #define they could overwrite when they go to prod?

[ed7coyne]

Alright lets try to make it configurable.

[proppy]

@ed7coyne also feel free to commit as it, if travis is happy about it and just file a bug to make it configurable.

[ed7coyne]

Sounds good, I will open a Issue for it.

Also unless you fixed it in your latest PR Travis is only actually testing the "Push" example. We should extend it to all of the examples.

[proppy]

Yes, since you're adding a new sample this PR should probably add it to the CI :)

[ed7coyne]

Added to travis.

[ed7coyne]

Merging PR based on previous approval.