itterates over an array, applying the callback for each item item. the callback takes the same style of arguments as `jQuery.each()` (index, item). The return value of each callback is collected as a single string and stored as `content` to be used in the next iteration of the `RenderContext`.
function(array, callback, now) {
      var context = this;
      var coll = function() {
        if (_isFunction(array)) {
          callback = array;
          array = this.content;
        }
        var contents = [], doms = false;
        $.each(array, function(i, item) {
          var returned = callback.apply(context, [i, item]);
          if (returned.jquery && returned.length == 1) {
            returned = returned[0];
            doms = true;
          }
          contents.push(returned);
          return returned;
        });
        return doms ? contents : contents.join('');
      };
      return now ? coll() : this.then(coll);
    }