code: function(key, callback) {
        if (!this.exists(key)) {
          return this.set(key, callback.apply(this));
        } else {
          return this.get(key);
        }
      }
docs: Returns the value at `key` if set, otherwise, runs the callback and sets the value to the value returned in the callback.  ### Example  var store = new Sammy.Store; store.exists('foo'); //=> false store.fetch('foo', function() { return 'bar!'; }); //=> 'bar!' store.get('foo') //=> 'bar!' store.fetch('foo', function() { return 'baz!'; }); //=> 'bar!