Added ASCII visualization of trees, modified from the disco-dop project by Andreas van Cranenburgh
Renamed `Tree` method `visualize` to `pretty_print`
Renamed the class `TreeVisualizer` to `TreePrettyPrinter`.
Renamed the file `treevisualizer.py` to `treeprettyprinter`.
Merge pull request #794 from heatherleaf/treedraw

ASCII visualization of trees