code: function throttle (fn, timespan) {
      var result;
      var lastCall = 0;
  
      return function () {
          var now = Date.now();
  
          if (now - lastCall >= timespan) {
              lastCall = now;
              result = fn.apply(this, arguments);
          }
  
          return result;
      };
  }
docs: Returns a function that will invoke the passed function at most once in the given timespan.<br/>
  The first call in this case happens as soon as the function is invoked; see also
  {@link module:lamb.debounce|debounce} for a different behaviour where the first call is delayed.
  @example
  var log = _.throttle(console.log.bind(console), 5000);
  
  log("Hi"); // console logs "Hi"
  log("Hi again"); // nothing happens
  // after five seconds
  log("Hello world"); // console logs "Hello world"
  
  @memberof module:lamb
  @category Function
  @see {@link module:lamb.debounce|debounce}
  @since 0.1.0
  @param {Function} fn
  @param {Number} timespan - Expressed in milliseconds.
  @returns {Function}