Tutorial draft (not finished) and documentation framework improvements

Tutorial draft (not finished) and documentation framework improvements

* a first step to a new documentation framework, using Markdown syntax, with
  some extensions (detailed in txt2myt.py docstrings):
  * `rel:something` for internal links
  * `{@name=something}` to override default header names (used when linking)
  * `{python}` to force code block to use Python syntax highlighting (not
     needed when using examples with `>>>` prompt)
* txt2myt.py -- converter from .txt to .myt
* a draft of tutorial.txt, which uses new syntax
* testdocs.py -- check examples in documentation using doctest (currently only
  in tutorial.txt)