function startPoller(interval, setTimeout) {
    (function check() {
      forEach(pollFns, function(pollFn){ pollFn(); });
      pollTimeout = setTimeout(check, interval);
    })();
  }
@param {number} interval How often should browser call poll functions (ms)
@param {function()} setTimeout Reference to a real or fake `setTimeout` function.

@description
Configures the poller to run in the specified intervals, using the specified
setTimeout fn and kicks it off.