[feature] Added update monitor that will set the system LED if updates are available.

added:
  src/update_monitor.cpp
  src/update_monitor.h
    - First version of an update monitor that will set the system LED if updates are available.

modified:
  src/mediasmartserverd.cpp
    - Added option --update-monitor (-u) for starting the update monitor.

  Makefile
    - Added lines for update_monitor.cpp.