Added check for czech ISBN.
Duplicite code moved to _validate_isbn().
Fixed bug in ISBN check.

_validate_isbn() didn't allowed blank series ISBN.
Small refactorings to improve readability. Added few comments.
Documentation updated.

Added lot of inter-document links.
Fixed typos.
Sphinx aplied to all visible methods.
Changed method of docstring sorting.
Documentation updated. Added Sphinx links to elements. Spellchecks.
Documentation for isbn.py updated.
Fixed documentation of __init__.py. Spellchecks in export.py.
Updated informations about project, titles of submodules in documentation.
Added "Related projects" section.