Adding a composer.json file for composer/packagist

### Composer and Packagist Integration

[Composer](http://getcomposer.org/) has been sweeping the PHP developer community for *finally* providing a capable package/dependency manager that's easy to implement and use.

This simple file allows **Klein.php** to be placed on [Packagist](https://packagist.org/) and easily downloaded/used in projects with a simple command-line tool.

All that's necessary is to:

1. Merge this pull request. :smile: 
2. [Register](https://packagist.org/register/) (or [login with GitHub](https://packagist.org/login/github)) on Packagist
3. [Submit](https://packagist.org/packages/submit) klein.php as a new package (REALLY easy)

*Optionally*: Setup a GitHub "hook" to automatically update the Packagist after pushing to master. :+1: 

This would be much appreciated by the community. I could do this myself, but I'd rather you @chriso have control over your own package/library.