Make barebones package layout

The package layout will be as follows:

The overall package is called gurps and it contains a subpackage space,
which will house the modules for the space system generation.
Move modules into package

Move the modules into the package gurps.space and change all import
statements to relative imports.

Close #11
Merge branch 'correct-packaging'

With this merge we start to adopt a classical pythonic package
structure, which should, in theory, help us later on when trying to
ship the code.