Firebase serial

[proppy]

A simpler API inspired by https://konekt.io/ that expose a Serial-port like interface over a firebase path.

• FirebaseSerial.begin("example.firebaseio.com/path/", "secret")
• String data = FirebaseSerial.read() will yield event data
• FirebaseSerial.print(data) will push data to path

We could differentiate between set() and push() w/ the termination of the path:

• example.firebaseio.com/path/ would append
• example.firebaseio.com/path would update in place

get() is a subset of stream() because the stream start by yielding event for existing values.

Also address part of #58, and could be a simple API variant for #35 and #66.

TODO

• handle trailing / to infer push vs set.
• buffer until \n
• merge path with url
• add readString, parseInt, and println(T) overload

/cc @aliafshar @ed7coyne @gguuss @mimming

[proppy]

Currently segfault if we do print() and read() in the same sketch, similar to what was reported with #48

[proppy]

Note: this doesn't crash anymore when using https://github.com/esp8266/Arduino master (esp8266/Arduino@5dd6acc).

[ed7coyne]

Comment needs updating.