composer support

Hi there,

I did some trivial modifications to change your IPN class to support composer. It breaks compatibility so you have to specify the namespace when creating an instance, like so:

$ipn = new Quixotix\IpnListener();

or, alternatively, you can add compatibility back to swaths of old files by adding the following at the top of an existing platter of spaghetti:

use Quixotix\IpnListener;

This kind of blows because you have to type more and change stuff, but it makes the class pretty much instantly interoperable with any PHP project in existance, and even those that don't exist yet. Also, you get to force other programmers to type your company name :)

So if you like, you can merge this, put out a new (backwards compatibility breaking) version tag like v2.2.0, and add the thing to packagist, and composer users can just use it in their projects by adding quixotix/paypalipn to their composer.json.

In any case, thanks for writing the lib, and have a great time!

Carlo