CMake: Print messages about Python configuration
Implement Pow::check_canonical() and use it
Implement Add:check_canonical()
Rename check_canonical() to is_canonical()
Add a test that currently fails

We comment the relevant part of Add::is_canonical() for now, until this gets
fixed.
Add another test to Mul::is_canonical()
Add another check
Improve a comment
Use static casts
Add 'coef' argument to Add::from_dict()
Fix the numeric handling of "x + 1"

Tests enabled.
Merge pull request #29 from certik/assert

Implements is_canonical() for Pow and Add