Merge pull request #4 from stoewer/refactoring

Project structure and PyBlock

Good to go! ;-)
.gitignore: add local Makefile to ignore list
proxy_list.py: A proxy that emulates a list

This proxy can use getter, setter and deleter of a NIX
entity in order to emulate a list like object.
block.py: a list like sources property for Block

Use ProxyList in order to implement a property for Block
that returns a list like object.
proxy_list.py: add __str__ and __repr__
PyBlock: add support for __str__ and __repr__