Release: don't build Python 3 tarball

Just build one tar.gz, it should work with all Pythons.
Document the logx parameter in Expr.nseries()

Add doctests. This parameter was introduced in
9c46974da12df56818020e29f76313fc0903ecc6 but it was never documented.
Docs updated
Intro and module structure completed.
logo: Upload the *.svg source of the logo.
logo: Implement the generation of logos of variouse formats and kinds.

Cleaning of *.svg file:

    - make the tail of the snake to be transparent.
    - remove xmlns:inscape and xmlns:sodipodi attributes.
    - insert the title and description.
    - determ, rearrange and comment the parts of of svg elements.
    - regroup the svg parts logically.
    - align SymPy text to its center.
    - set a size of logo picture to be square.
    - upper Integral on the left edge of cube

Generation of files:

    ./bin/generate_logos.py --all
logo: fix typo
logo: Update logo.txt
logo: Rename logo.txt and merge it with LogoInfo.txt.
logo: fix alignment of integral symbol and '['.
logo: reorganisation to use Makefile.
logo: Try Sigma to be scaled in Mac OS.
logo: Use Comuter Modern fonts.
logo: create version where text is path.
Create info.txt.
Fix logo-info.txt: dos -> unix fileformat
Minor formatting fixes in generate_logos.py
Remove some ambiguous short options for generate_logos.py
Fix logo target in doc/Makefile
Autogenerated files are now in doc/_build/logo/; fix logo-info.txt accordingly