Improving Cosa Nucleo; Thread and Semaphore. Adding Mutex support class.

1. The Nucleo main thread will power down (SLEEP_MODE_IDLE) when threads
are waiting and there is nothing to do.
2. Semaphore::signal will resume the first waiting thread if any.
3. A new Mutex class adds some syntactical sugar to Semaphore usage.
4. Example sketch for Mutex; CosaNucleoMutex.ino.