Add Arduino compatibility layer

We can now directly compile arduino sketches (.pde) files.
Arduino compatible analogWrite works now.
But there is still a long way to go, serial I/O and timer stuff (delay,
millis etc) currently don't work (not tested but I don't expect this to
work).
It can be used in an arduino sketch or in a normal contiki program.
We get a PWM frequency of 490.2 Hz (a period of 2.040 ms), that's
Arduino compatible. If you need different frequencies see native timer
usage in examples/osd/pwm-example
In a contiki program you have to call arduino_pwm_timer_init to
initialize the timer before pwm works. The arduino sketch wrapper
already does this.
For running a sketch, see examples/osd/arduino-sketch

examples/osd/arduino-sketch/README.md

@@ -0,0 +1,11 @@
+Arduino compatibility example
+=============================
+
+This example shows that it is now possible to re-use arduino sketches in
+Contiki. This example documents the necessary magic. Arduino specifies
+two routines, `setup` and `loop`. Before `setup` is called, the
+framework initializes hardware. In original Arduino, all this is done in
+a `main` function (in C). For contiki we define a process that does the
+same.
+
+See the documentation file in apps/contiki-compat/README.md