Add Composer support

Adds basic [Composer](http://getcomposer.org) support. The details in the `composer.json` may need some tweaking before submitting to [Packagist](https://packagist.org/).

The Autoloader was removed in lieu of Composer's standard built-in, and also the bootstrap.php file from inside the `src/` directory (such a file should reside in the root directory, if deemed necessary).