Organize code according to PSR-0

This PR re-organizes the code to adhere to [PSR-0](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md). This is a required to include this project as a dependency into other projects (more PRs to follow to give some more insight). It also vastly simplifies working with the source code, because each class is in an independent file.

It does *NOT* change any of the existing code.