Add basic functions to create simple diagram in Python code.

[KrzyHonk]

At this point, implement adding only the simpliest elements of BPMN diagram:

• task,
• start event,
• end event,
• inclusive and exclusive gateway,
• sequential flows.

Possible obstacles and problems:

• generating id for new elements, provide unique values,
• identifying the nodes connected by flow (edge),
• Diagram Interchange informations - we can't expect the user to input them directly, so those have to be generated automatically.