- add a new sphinx extension "viewsource". takes advantage of part of the viewcode...

- add a new sphinx extension "viewsource".  takes advantage of part of the viewcode extension
to allow ad-hoc display of the source of any file, as well as a "directory listing" structure.
- reorganize examples/ to take advantage of new extension.  in particular, keep moving all
the descriptive text for files etc. into module docstrings, taking more advantage of
self-documentation.