add support for reusable architectural tests

Spoon is great for writing architectural tests, see http://spoon.gforge.inria.fr/architecture_test.html

Some tests we write could actually be useful for lots of Java projects, such as the recent one that checks for unused fields.

We could package the reusable architectural tests in some way so that we can share them with the world.

List of already implemented reusable architectural tests:

* all classes are documented with a documentation of certain length
* all methods are documented with a documentation of certain length
* don't use TreeSet without a comparator
* all tests follow the surefire convention
* all public methods of the API are directly called in a test
* helper classes only have static methods and a private constructor
* allowed packages are specified
* all fields are read
* all private methods are used