Fix Makefile typo
Update MANIFEST.in
Fix bug #21: arg_set should return frozenset
Avoid inheriting expr.Variable from boolfunc.Variable

Per issue #20, we need a way to implement Variables as tables, BDDs,
etc, while keeping the boolfunc.Variable instance a singleton.

The solution is to just use composition instead of multiple inheritance.
Now the new expr.ExprVariable class *contains* an instance of
boolfunc.Variable.
Simplify Variable/Complement support property
Add some initial BDD framework
Simplify calculation of a variable's uniqid

I have avoided using any variation of "identity" for the unique integer
variable identifier, to avoid confusion with the Boolean identity: 1 for
And, and 0 for Or.

The property 'uniqid' seems to capture the essence, so let's go with
that.
Use VARIDS in NormalForm expression

After tweaking the set operations in NormalForm.restrict, DPLL sat
is significantly faster. Looks like about 20-30%.
Get rid of unnecessary frozenset constructor
s/VARIDS/VARIABLES
Update boolfunc top comment
Implement expr2bdd function and unique table
Implement expr2bdd function and unique table
Add support/inputs properties to BDD
Fixup Expression/NFExpression variable usage

A few commits ago, I made it so ExprVariable does not inherit from
boolfunc.Variable. This turned out to be a mistake. Aside from fixing
that, I did a bunch of refactoring of how uniqids are used.
Use nifty str.zfill method
Merge remote-tracking branch 'upstream/master'
Add a simple example to play with bdd
Merge branch 'master' of https://github.com/rbarzic/pyeda