Add tinyfsm, start converting core functions to an FSM-based event loop

lib/tinyfsm/doc/20-Installation.md

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+Installation
+============
+
+TinyFSM is an header-only library, no special installation steps are
+needed. Just point your compiler to the "include" directory, and in
+your source files:
+
+    #include <tinyfsm.hpp>
+
+
+Prerequisites
+-------------
+
+TinyFSM requires a compiler supporting the C++11 language standard
+("-std=c++11" in gcc).
+
+TinyFSM does not depend on RTTI, exceptions or any external library.
+If you need to compile without standard libraries (e.g. in conjunction
+with `-nostdlib` linker option), add `-DTINYFSM_NOSTDLIB` to the
+compiler options: this removes all dependencies on the standard
+library by disabling some compile-time type checks.
+
+
+Building the Elevator Example
+-----------------------------
+
+Change to the elevator example directory and compile the sources:
+
+    $ cd examples/elevator
+    $ make
+
+Our elevator has call buttons on every floor, sensors reporting the
+current position, and an alarm button for emergency. These actors can
+be triggered via a simple command interface:
+
+    $ ./elevator
+    Motor: stopped
+    Motor: stopped
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ?
+
+Let's call the elevator to floor 2:
+
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ? c
+    Floor ? 2
+    Motor: moving up
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ?
+
+Now the elevator is moving up, and we need to trigger the floor sensor:
+
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ? f
+    Floor ? 1
+    Reached floor 1
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ? f
+    Floor ? 2
+    Reached floor 2
+    Motor: stopped
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ?
+
+Now we simulate a sensor defect:
+
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ? c
+    Floor ? 1
+    Motor: moving down
+    c=Call, f=FloorSensor, a=Alarm, q=Quit ? f
+    Floor ? 2
+    Floor sensor defect (expected 1, got 2)
+    *** calling maintenance ***
+    Motor: stopped