All classes now define their own dependencies.

This siplifies tests that only need to include the class
they want to test and not worry about including all the
dependencies too, as these will be automatically included
by the class itself.

Additionally the header of each file gives a nice overview
of the direct dependencies each class has.

I copied the previous lump-all-together-into-main-file-design
from some ruby project in github, thinking there is some value
in doing so.  Well there wasn't...