- All special cases for negation have been removed, except when they are also
  present in the parser.
- Some callbacks have been added to be able to customize output for certain
  operators in an extension of the Node class. This prevents chaotic sceneries
  in the graph_drawing library.
- Some useful unit tests have been added to test the precedence- and
  callback-meschanisms.

Conflicts:

	line.py

This reverts commit 821c5062.

This allows for overwriting of the stringification method. To be able to
introduce utf-8 version of the PI-character, for example.