Further reworked the connections and couplings.
Gave subsystems their own DocumentObject. Component and Connection use now DocumentObjectGroupPython instead of FeaturePython, making it possible to group objects under them (respectively Subsystems and Couplings). Note that this is currently broken in FreeCAD GUI.
Updates to documentation.
Merge pull request #30 from FRidh/master

Redesigned Connections, Couplings and Subsystems