modified node.py and tree.py : improved documentation

o added a license header to each file
o added at the beginning of each file a list of the classes
o modified some details required by Pylint : e.g. 'return' and 'del' keywords are to be written without parentheses
o Tree and Node classes begin with a list of methods, attributes and class attributes
o almost all lines have a length lesser than 80 characters, except in the license text
o added a very brief documentation to error classes

o many functions have one or two lines of documentation. This documentation is very basic for the moment
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.
modified tree.py and node.py :

as benoitbryon suggested me :
* replaced first line by """treelib - Simple to use for you.
* removed useless doc in classes and methods
* replaced this useless doc by annotations to be read by Sphinx, using the "#:" syntax
added AUTHORS file
Merge pull request #29 from suizokukan/master

modified node.py and tree.py : improved documentation