Add photoset_layout as an option to create_photo

Even though this is not a documented feature of the Tumblr API, it works well, and this attribute is actually provided by their Posts endpoint (http://www.tumblr.com/docs/en/api/v2#posts)

Allowing this `photoset_layout` option as a valid attribute to create an image post would be a nice addition since it gives full control over how photos are laid out in the template.