PSR-2 conversion and Composer installation support

PHP libraries are advised to comply with the [PSR-0 standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md) for autoloader interoperability.

Also, the de-facto standard for PHP dependency management is [Composer](http://getcomposer.org/).

This PR moves the Phaxio classes in a `Phaxio` namespace, and makes the code compatible with [PSR-0](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md), [PSR-1](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-1-basic-coding-standard.md) and [PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md).

It also adds a `composer.json` file for Composer.
You can then [submit the package](https://packagist.org/packages/submit) to Composer (this PR will need to be accepted first).