Legacy support

@drsnyder 
Adding "scale-to-width" mode, which creates a thumbnail with the given width, scaling the height to maintain the aspect ratio. Many legacy urls are in this format, and it's easy enough to implement. Also returning a [307](http://en.wikipedia.org/wiki/List_of_HTTP_status_codes#3xx_Redirection) response for unsupported legacy paths, which for now include paths with custom window cropping with the offset url parameters (ex: 200x200-0,200,0,150). We'll have to add a varnish rule somewhere to redirect this to the old thumbnailer.
What does `:height "0"` mean here?