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/sketch.pde

@@ -0,0 +1,47 @@
+/*
+ * Sample arduino sketch using contiki features.
+ * We turn the LED on and off and allow setting the interval and the
+ * brightness of the LED via coap.
+ * Unfortunately sleeping for long times in loop() isn't currently
+ * possible, something turns off the CPU (including PWM outputs) if a
+ * Proto-Thread is taking too long. We need to find out how to sleep in
+ * a Contiki-compatible way.
+ * Note that for a normal arduino sketch you won't have to include any
+ * of the contiki-specific files here, the sketch should just work.
+ */
+
+extern "C" {
+#include <stdio.h>
+#include "led_pwm.h"
+#define LED_PIN 5
+
+uint8_t pwm          = 128;
+uint8_t period_100ms = 10; /* one second */
+}
+
+void setup (void)
+{
+    rest_init_engine ();
+    rest_activate_resource (&resource_led_pwm);
+    rest_activate_resource (&resource_led_period);
+}
+
+void loop (void)
+{
+    static uint8_t last_pwm = 0;
+    if (last_pwm != pwm) {
+        last_pwm = pwm;
+        analogWrite (LED_PIN, pwm);
+        printf
+            ( "TCNT3: %04X TCCR3A: %04X TCCR3B: %04X TCCR3C: %04X OCR3C: %04X\n"
+            , TCNT3, TCCR3A, TCCR3B, TCCR3C, OCR3C
+            );
+    }
+
+    // Originally I wanted to sleep here to make the LED blink.
+    // Sleeping currently doesn't work, something turns off the chip.
+    // Maybe a mechanism to guard agains proto-threads taking too long?
+    //clock_wait (CLOCK_SECOND * period_100ms / 10);
+    //analogWrite (LED_PIN, 0);
+    //printf ("After write\n");
+}