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/led_pwm.h

@@ -0,0 +1,30 @@
+/**
+ * \defgroup Arduino LED PWM example
+ *
+ * Resource definition for Arduino LED PWM module
+ *
+ * @{
+ */
+
+/**
+ * \file
+ *         Resource definitions for the Arduino LED PWM module
+ *      
+ * \author
+ *         Ralf Schlatterbeck <rsc@tux.runtux.com>
+ */
+
+#ifndef led_pwm_h
+#define led_pwm_h
+#include "contiki.h"
+#include "contiki-net.h"
+#include "erbium.h"
+#include "er-coap-13.h"
+
+extern uint8_t pwm;
+extern uint8_t period_100ms;
+extern resource_t resource_led_pwm;
+extern resource_t resource_led_period;
+
+#endif // led_pwm_h
+/** @} */