remove sectnum directive from source files

as recommanded by sphinx http://sphinx-doc.org/rest.html#directives
remove sectnum
replace by :numbered: parameter in the top-level .. toctree::
Merge branch 'master' of https://github.com/bneron/E2I2B.python
fix typo recommand -> recommend
add example to illustrate cProfile usage

add optimization_time.py script
and output of cProfile

modify css to change the backgound color of emphasize-lines
of code-block text directive