Fix bug with ITE factoring
Always simplify before factoring
Use p, q instead of antecedent, consequence
Fix issue #15

You can invert a mux using the following logic:
f = s ? a : b
-f == s ? -a : -b
Fixup invert methods

If an expression is not simplified, don't simplify its negation.
Normalize all checks for expression in B = {0, 1}
Fixup Complement.restrict
Use tuple for Table inputs
Add Shannon expansion capability to Expression
Add Function reduce method
Hack for Python 2.7 issue