Define autoload for Composer.

As I mentioned [here](https://github.com/michelf/php-markdown/pull/42#issuecomment-9515517), the `composer.json` needs to have autoload information in order for Composer to actually be able to autoload the classes.

It would probably be good to have a new beta2 tag as soon as this goes in as anyone who asks for `@beta` or `minimum-stability: beta` is going to have a problem with the existing beta1 version.

It would probably be good to test an install from Packagist using `@dev` after this change is merged to make sure that there are not other issues *before* doing the beta2 tag, though.

I'm happy to help! Excited to start directing people here over my fork. :)