Add Composer support

Hey Craig!

I decided to take up on [your offer](https://github.com/ccampbell/chromephp/issues/22#issuecomment-12505229) and write a composer.json file for your class! Please check the data and see if everything is OK (especially in the description and authors and such).

Once you merged my changes, I'll submit the package [here](https://packagist.org/packages/submit). Composer is a dependency manager for PHP, it allows you to manage libraries and packages on a per-project basis. If you still do work with PHP you should check it out. :)