title: How to set time for scanning iBeacon when calling RangeNotifier?
body: <p>Normally, it takes 1sec to have a new scan.<br>
  If the scanning is 1sec, I may not do too much things if I want to parse the data.<br>
  How to change that value?</p>
  
  <p>Also, for the background service, how to change the value of freq ?</p>