Make compiler use Sk.builtin.none object instead of null for 'None' in Python.
Make compiler use Sk.builtin.bool instead of Javascript booleans for Python values.
Clean up richCompareBool and get rid of unnecessary code.
Small changes to lib modules to make compatible with Sk.builtin.none.
Merge branch 'master' of github.com:rixner/skulpt into rixner-master