Problem: class config properties are poorly named

Solution:

* class.header -> class.package_dir
* class.source -> class.source_dir
* class.include -> class.project_header
* class.proto -> class.protocol_class
* class.export -> class.export_macro

This breaks existing models, sorry. The new names are nicer though.
Merge pull request #114 from hintjens/master

Problem: class config properties are poorly named