Fix default SAT algorithm choices
Create 0.12 release notes
Release 0.12.0
Fix issue #19

The literature indicates the "top" variable of a function is the one
with the *lowest* index in the ordered support set.

Just in case it's useful, added a "bottom" variable.
Fix issue #19

The normal form expressions had too many conversion methods. Now they
are consolidated into just expr2nfexpr and nfexpr2expr.
Add basic example of pyda usage to check equivalence between boolean functions