added a AUTHORS file

modified tree.py, node.py :
o flake8 is now ok with modifications of my own but it remains two problems in tree.py, lines 655 and 727 (E127 continuation line over-indented for visual indent)
o arguments in a "def function()" line are now either on one line either or two lines (80 characters/line)
o removed the "ASCII style" separators like "#////////////..."
o removed the license header at the beginning of each file

By the way, I didn't follow Natim's suggestion to change the date in the copyright (Natim's words : "should be 2012-2013"). I let this modifications to other people since I wasn't sure what I had to write.