add ExampleASTVisitor:

* prints out examples of nodes that are handled by visitor.  simply a
  development convenience

remove NestedCodeGenerator -- it was bogus after all
replace with generateFunctionCode, a method to call to generate code
  for a function instead of a top-level module

fix impl of visitDiscard (most pop stack)
emit lineno for pass

handle the following new node types: Import, From, Getattr, Subscript,
Slice, AssAttr, AssTuple, Mod, Not, And, Or, List

LocalNameFinder: remove names declared as globals for locals

PythonVMCode: pass arg names to constructor, force varnames to contain
them all (even if they aren't referenced)

add -q option on command line to disable stdout