Allow URLcrypt to be used as Composer package

I added a composer.json and enabled PSR-4 autoloading via Composer. I introduced namespaces and made testing a little bit easier. If you want to release this as Composer package you have to add your e-mail address to the composer.json file and you should be almost ready.

I also recommend you to delete the tag v0.1.0 because it points to the original Ruby version.