Add Composer installation support

[Composer](http://packagist.org/) is the new emerging default way of handling PHP library dependencies. This patch adds support for it that works very similarly to the NPM support in the Node.js buildpack: if there is a `composer.json` file in an app, then its dependencies are installed with Composer.

Here is a simple example app using it:

<http://urlizer-service.herokuapp.com> ([source code](https://github.com/bergie/urlizer_service))