The aim of this module is to provide a common base representation of
python source code for projects such as pychecker, pyreverse,
pylint... Well, actually the development of this library is essentially
governed by pylint's needs.

It extends class defined in the compiler.ast [1] module with some
additional methods and attributes. Instance attributes are added by a
builder object, which can either generate extended ast (let's call
them astng ;) by visiting an existant ast tree or by inspecting living
object. Methods are added by monkey patching ast classes.

Main modules are:

* nodes and scoped_nodes for more information about methods and
  attributes added to different node classes

* the manager contains a high level object to get astng trees from
  source files and living objects. It maintains a cache of previously
  constructed tree for quick access

* builder contains the class responsible to build astng trees
