Implements results of next.parallel as dictionary

This pull request seamlessly integrates a dictionary behaviour to next.parallel as described below.

    common.step([
      function (next) {
        next.parallel('key')(null, 'value');
      },
      function (results) {
        var key = results.key;
        console.log(key);
      }
    ]);

We think this helps maintain some sanity when doing lots of parallel activity.