Fix display thumbnails

Something happened to this function along the way and it stopped displaying thumbnails. `display_post_thumbnail_src()` was creating an array of urls to thumbnails in the post, and `display_post_thumbnail()` had a mysterious `$src` param with a value that could never be equivalent to the output of `display_post_thumbnail_src()`. So it displayed nothing. We just need one image url or the `DEFAULT_PHOTO` constant returned from `display_post_thumbnail_src()`, and `display_post_thumbnail()` just needs to check if `display_post_thumbnail_src()` has a value and display it along with the right class. I think we were trying to be too fancy.
You still need the $link var above, otherwise this line spits out an error.

![screen shot 2014-07-31 at 8 18 46 pm](https://cloud.githubusercontent.com/assets/1280430/3773560/7b23be10-1911-11e4-8ec6-090091f10554.png)