Add a basic support for automatic table of content generation thanks to a macro-like syntax: <<toc>>

I added a toc_test methods to TestCreole2htmlMarkup to perform unittest. I pached the
existing tests so that they are correct despite the modification I made.

It supports:
- max depth: depth=NUMBER
- links to the correct section
Various code improvment:

- Avoid use of float('inf')
- Rename python_list2html_list to toc_list2html (more coherent)
- Rename variables in toc_list2html to be clearer
- Use document.replace('<p><<toc>></p>', html_toc, 1) in toc emit to limit replacement

Unit tests still fail in python2 (but pass with python3).
Fix the unitary tests.
Should fix travis error.
Merge pull request #19 from Jenselme/toc

Add a basic support for automatic table of content generation