StringTree.__getitem__() returned the node, now return its value
child instead. That's nicer and closer to the expected behaviour, as
it supports StringTree usage as a key-value store for configuration
or so. It breaks the existing API, but I think I have fixed the only
pace where it was used: jw-grammar/grammar.py.
Signed-off-by: Jan Lindemann <jan@janware.com>