Fix callback not fired

[DrJest]

Fixes an issue with callback not being fired if there are not enough post in the feed to reach the limit. Also fixes an error if pinterest response is empty

[nicosuria]

Found an issue where the loaded_post_count wasn't getting incremented in case the feed.render() call failed, thus failing the equality check between it and posts_to_load_count when attempting to call fireCallback. Example: if one of the posts fetched was a duplicate.

Fixed it by adding the following (using twitter as an example)

// js/jquery.socialfeed.js line 264

var post = new SocialFeedPost('twitter', Feed.twitter.utility.unifyPostData(element));
var render_successful = post.render();
// If we do not render the post because it was a duplicate or something, reduce the
// completion counter so that the `loaded_post_count` and `posts_to_load_count` resolve to equal

if(!render_successful)
  posts_to_load_count--

[HartLarsson]

there was already a fix that works perfectly, for example for twitter https://github.com/pavelk2/social-feed/pull/173/commits/1fe5903edff00946684fb210d5ac1afd6540aa90:

                    getPosts: function(json) {
                        if (json) {
                            // fix callback render
                            if(json.length<options.twitter.limit) posts_to_load_count -= options.twitter.limit - json.length;
                            $.each(json, function() {
                                var element = this;
                                var post = new SocialFeedPost('twitter', Feed.twitter.utility.unifyPostData(element));
                                post.render();
                            });
                        }
                    },