Changed the implementation of the test decorators
renamed file
you can't use two different decorators any more
Refactoring of __main__
added some documentation
Starting to refactor __main__ into more object-oriented code
Deleted plugin list builder tests

PluginListBuilder and its collaborators simply implement an
algorithm from graph theory. So they're unlikely ever
to change. And in any case, the tests can always be recovered
from the history