adding a loading spinner, mostly

[paulirish]

• we use a data uri loading spinner GIF that is added to the controls bar while the image loads.
• the convert.php on slideshare that this runs through seems to be VERYYYY slow but once the slides are in their image cache, they seem to be fine.. not much we can do about that.
• i also hardcoded the aspect ratio for the slides.. sometimes a failed image meant the controls jumped to the top.. i've also seen an image go wayyy too high. so now we enforce a height on the image. i don't see where this will be a problem, but let me know if you think so.

thanks again for this project! i love it.

[codepo8]

You spin me right round...