Added a timer

firmware/drivers/ptimer/ptimer.cpp

+#include <ptimer.hpp>
+
+unsigned long PTimer::start_t_ = 0;
+
+void PTimer::start(){
+    start_t_ = micros();
+}
+
+unsigned long PTimer::elapsed(){
+    return static_cast<unsigned long>(micros - start_t_);;
+}

firmware/drivers/ptimer/ptimer.hpp

+#ifndef PTIMER_HPP_INCLUDED
+#define PTIMER_HPP_INCLUDED
+class PTimer{
+public:
+    static void start();
+    //returns the elapsed time in microseconds
+    static unsigned long elapsed() const;
+private:
+    static unsigned long start_t_;
+};
+#endif