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/proxmark3.c

@@ -22,7 +22,6 @@
 #include "cmdmain.h"
 #include "uart.h"
 #include "ui.h"
-#include "sleep.h"
 #include "cmdparser.h"
 #include "cmdhw.h"
 #include "whereami.h"