This is going to be a bit more comprehensive than anticipated. In order
to cleanly refactor the core and the engine, basically start from
scratch with the implementation.
Goals:
* Test coverage
* Maintain backwards compatibility with module interface as much as
possible (but still make modules easier to code)
* Simplicity
see #23
Add a very simplistic framework for loading modules that query system
data. The user can provide a list of modules via an argument switch, and
the name of the module is used to look up a Python module that has to
have a class called "Module".
The outer framework (the outputs, in particular) then uses various query
methods of that class to construct a meaningful output.
