Provide msclock() as Milliseconds timer for performance measures (#231)

- don't use clock(). It has different functionalities in Windows and Linux
- move sleep functions to util.h

client/hid-flasher/proxusb.c

@@ -18,7 +18,6 @@
 #include <strings.h>
 #include <errno.h>
 
-#include "sleep.h"
 #include "proxusb.h"
 #include "proxmark3.h"
 #include "usb_cmd.h"