function throttle (func, wait) {
  var rtn; // return value
  var last = 0; // last invokation timestamp
  return function throttled () {
    var now = new Date().getTime();
    var delta = now - last;
    if (delta >= wait) {
      rtn = func.apply(this, arguments);
      last = now;
    }
    return rtn;
  };
}
Returns a new function that, when invoked, invokes `func` at most one time per
`wait` milliseconds.

@param {Function} func The `Function` instance to wrap.
@param {Number} wait The minimum number of milliseconds that must elapse in between `func` invokations.
@return {Function} A new function that wraps the `func` function passed in.
@api public