New feature: maxCallsCallback

[nickpearson]

I needed a way to be notified when PeriodicalUpdater stopped checking a remote URL due to the maxCalls being reached. I added the following line, which goes at the end of the getdata function.

function getdata(force) {
  // ...
  if (force || maxCalls === 0) {
    // ...
  } else {
    pu_log("NOT sending...");
    // added the following line (like autoStopCallback)
    if (settings.maxCallsCallback) { settings.maxCallsCallback(); }
  }
}

This works for my needs, but I didn't want to do a pull request since you may want to further refine it and/or call it something different.

[RobertFischer]

Just pushed it: e2a88e2