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.

to use declarative, added a new example
dict_of_sets_with_default.py, a "pushing the envelope"
example of association proxy.

- added READMEs to all examples in each __init__.py and added to sphinx documentation
- added versioning example
- removed vertical/vertical.py, the dictlikes are more straightforward