add sample for firebase modem

[proppy]

Revive the mini-UART protocol we had in 566c303#diff-dc3ebc3d309c06217829e6e5ed607be0R71

With the following commands:

> PUSH /path {"some json":"blob"}
{"some json": "id"}
> UPDATE /path {"some json":"blob"}
{"some json": "id"}
> VAL /path
{"some json":"blob"}
> STREAM /path
{"some json":"event"}
{"more json":"event"}

[ed7coyne]

So thinking towards the actual title of this issue.

I created an example that demonstrates the useage of our serial protocol:
https://github.com/ed7coyne/firebase-arduino/blob/serial-example/examples/FirebaseSerialClient/FirebaseSerialClient.ino

But I didn't do a pull request because I don't think it is actually a compelling use-case. I think we should create something actually interesting.

What ideas can we come up with?

One I had was: Create a Sender module that has a photosensor and a thermometer that can be placed outside. Create a Corresponding Receiver module that has a Neopixel attached to it. You can then place the Neopixel inside somewhere and it will vary brightness to match outside light conditions and color from red to blue to match temperature. Between these sensors we could demonstrate SET,GET and STREAM.

[proppy]

Related #84

[ed7coyne]

I think the sample we have addresses this issue.