Make Composer compatible, add Twig convenience functions

I'm quite keen to get this package on Packagist, so I've set this up as a "fuel-package" to retain the correct fuel structure and updated the readme to correlate. Also added a couple of convenient Twig functions.

For now I'll use the forked version as "soundintheory/fuel-sprockets" so there is no rush, but I was just checking if this may be useable for you to get the "real" package added.

Great work by the way!