Add pscss as bin in composer.json

This *really* simple patch adds the `pscss` as a [bin](http://getcomposer.org/doc/04-schema.md#bin) in composer.json.

Other projects consuming scssphp will thus receive a `vendor/bin/pscss` file as expected.

Besides, I also needed this in order to automatically bundle scssphp as a phar (https://github.com/clue/phar-composer).