Split chapters, improved navigation and self reporting of user progress

Many file changes since source contains new rst files (a separate rst file for each topic of a chapter). 
A new toc.rst, based on the new structure of the book.
A backup of the old source folder is available in source_original. 
The chapter Classes is split into ClassesBasics and ClassesDiggingDeeper, to maintain a consistent hierarchy or chapters.

A new configuration step is added to building of the book. See step "Populate chapter tables" in the main Readme.rst. 
A new Readme.rst file is created in runestone/thinkcspy for the script chapterNames.py.

New feature, which enable user to self report completion of a section. The TOC page, gives an overview of the overall progress. 
The feature "You were last reading" was lost in the new design. Reintroduced.