[antlr-interest] added tree-based interpreter tutorial

[Terence Parr]

Added a tutorial, which was impetus for  a bunch of changes for 3.0b6  
(which have been pushed to the depot, you can grab the latest from  
the repository browser).

Couldn't really build a tree-based interpreter before, but now you  
can as demonstrated in the tutorial:

http://www.antlr.org/wiki/display/ANTLR3/Simple+tree-based+interpeter

Ter