Fix image gallery list (overlapping images)

This fixes https://github.com/elcodi/bamboo-admin/issues/17

There are 2 main reasons: JS errors that prevented rest of the JS code to be executed, and masonry applied before images were loaded, and proper height was calculated. To avoid this second problem, I added imagesLoaded (http://imagesloaded.desandro.com from the same author as Masonry)  as explained here http://masonry.desandro.com/appendix.html#imagesloaded