Enhances thumbail handling

I did the mistake of taking a look at the thumbnail code and simply couldn't help doing at least some refactoring to make it a bit less terrible:
 - Removes duplicate image handling code
 - Serves the images without loading the engine
 - Makes the code more readable

What do you think of the approach?

BTW, we allow users to define ```access_id``` for the plugins, so should the images also respect it? I find it tempting to serve the images without checking the read access in ```image.php```.
Getting the path from ```SCRIPT_FILENAME``` allowed this to work in my dev environment where the plugin is located outside Elgg root. I guess it doesn't hurt to use it in production also?