Merge pull request #275 from Boldie/master

Implementing a way to print labels as PDF
Moves the abstract renderer out of the firectory with the renderer to prohibit warnings from the renderer class finding mechanism.
Removes the require_once from the RendererFactoryRegistry and also removes the global execution code.

Global execution code is a source of problems and since we have found a
clever way to use the class loader for finding our classes we can now remove
this code. Furthermore we added more error checking for plugin classes
and will warn the user about its error.
Adds comments to make build system happy. Additionally some comments are improved.