Add some BDD demonstration IPython code
Add some BDD documentation
Move images into repository

This should fix the warning about non-local URI,
and hopefully PDF generation will work correctly now.
Fix boolexpr.parse docstring
Improve the parse docstring some more
Get rid of unnecessary import
Move assumptions to first argument of satisfy_one
Allow arbitrary, nonzero assumptions
Improve parse_cnf docstring and error handling
Add basic picosat script
Cleanup some incorrect code-block usage
Add a section of the PicoSAT script
Minor cleanup
Add a bunch of new BDD documentation
Update BDD notebook with examples
Use pixels for height units
Fixup BDD weakref garbage collection

The _RESTRICT_CACHE was creating some kind of circular reference, which
prevented BDD nodes from getting automatically garbage collected.
This change makes the restrict cache local to the _urestrict call.