Allow java implementation to load assets from filesystem

A simple patch to let java version to load assets from one or multiple directories using a new Assets implementation when "assetsDirectories" new setting is set.
What are you looking to catch with this? The MalformedURLException thrown by f.toURI().toURL()? If so, be specific  about that(and ideally, narrow the try/catch block to where it's really needed). As it stands, you'll catch the FileNotFoundException thrown a line above & re-throw it, which is weird.