add optional verbose arg to walk function. it overrides the global

VERBOSE setting for the ASTVisitor

add getopt handling for one or more -v args

rename ForwardRef to StackRef, because it isn't necessarily directional

CodeGenerator:
* add assertStackEmpty method.  prints warning if stack is not empty
  when it should be
* define methods for AssName, UNARY_*, For

PythonVMCode:
* fix mix up between hasjrel and hasjabs for address calculation