Add composer.json to allow installation via composer

It would be really helpful to be able to install the Kohana coding standards via composer so that they can be used easily in both Kohana and non-Kohana projects. I've provided and tested a composer.json and updated the readme to cover this. I wasn't sure who to list as developer/maintainer so I've just specified the Kohana team but I can easily change that if you wish.

If you accept this patch then the only remaining step would be to register the package on packagist - I'm happy to do that. After that Packagist will keep itself up to date so there shouldn't be any ongoing maintenance required.