code: function(path, interval, fn) {
  	if(typeof fn === 'undefined') {
  		fn = interval;
  		interval = 100;
  	}
  	if(typeof interval !== 'number') return false;
  	if(typeof fn !== 'function') return false;
  
  	var value;
  	var readTimer = setInterval(function() {
  		_read(path, function(val) {
  			if(value !== val) {
  				if(typeof value !== 'undefined') fn(val);
  				value = val;
  			}
  		});
  	}, interval);
  
  	this.stop = function() { clearInterval(readTimer); };
  }
docs: fs.watch doesn't get fired because the file never gets 'accessed' when setting header via hardware manually watching value changes