code: function watch (poet, callback) {
    var watcher = fs.watch(poet.options.posts, function (event, filename) {
      poet.init().then(callback);
    });
    poet.watchers.push({
      'watcher': watcher,
      'callback': callback
    });
    return poet;
  }
docs: Sets up the `poet` instance to watch the posts directory for any changes
  and calls the callback whenever a change is made
  
  @params {Object} poet
  @params {function} [callback]
  @returns {Poet}