Update package to use PSR-4

This PR updates the package to use [PSR-4](http://www.php-fig.org/psr/psr-4/).

Doing so gives a significantly flatter package structure, and makes setting up unit testing far simpler; unit tests can be run from the root of the package now with no issues. Additionally, the ZF2 `StandardAutoloader` already provides PSR-4 style support, so a slight modification to the `getAutoloaderConfig()` means that this module will still function properly for developers not using Composer.

The only potential stumbling block is for developers who are running an out-of-date Composer install; however, since Composer nags for installs older than 30 days, I'm not terribly concerned about that.