Split chapters, improved navigation, user progress

Two scripts have been created to split chapters:
1. thinkcspy/split.py - creates a copy of the source folder to
original_source and splits each rst file to multiple rsts per
subchapter.
2. thinkcspy/chapterNames.py - populates database tables chapters and
sub_chapters by scraping toc.html
Now, users can mark a subchapter as completed. You can see an overview
of the progress on the toc.html page