Load sprintf.js as git submodule

At first I thought you simply forgot to add the javascript file for sprintf. So I added it with a git submodule. But you already managed this with the help of a make file. :+1: 

Nevertheless, here is my patch to use git submodule. Feel free to refuse it :)
But in the long run, imho I think its easier to manage dependencies with submodules instead of custom scripts with wget/curl commands.