Implement sub(a, b)
Enable calling Mul::from_dict() with empty dict
Test a bug in "x - x"
Add a test for test_sub()
Implement "-" operation in Python wrappers

And test it. Two tests that don't work yet are commented out.
Fix a 3xy+xy bug and write a test
Fix a bug in -1*x*y
Enable two Python tests that work now
Merge pull request #34 from certik/arit

Implement the "-" operation