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

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