- syntaxtree.py: context and structure now properties of Node objects + switched str() and .context semantics

- transform.py: syntax-tree traversal now passes not only the current node but the complete context (i.e. list of all parent nodes) to the transformation function.
- similarly parser.Compiler now stores the context (i.e. list of all parent nodes) in self.context.
- more documentation for parser.Compiler, parser.Parser and parser.Grammar