Change Namespace to handle null values as Python does for None

[hpaulj]

TestPositionalsNargsOptional adapted from test_argparse.py gives an error
'AssertionError: {} deepEqual {"foo":null}'

A dozen other classes in the py test file give similar errors.
The problem is that the JS Namespace class treats null differently than the python None.

Python gives every dest at least a None value.
The JS Namespace tries to treat a null value as the same as undefined.
However, because sometimes values are set with Namespace.set(), and other
times with ordinary object assignment, sometimes a null value appears in
a namespace.

These tests do pass if a modified deepEqual is used, one that ignores
differences between {bar: null} and {}

Python has a special Namespace class because its base Object class does
not directly accept attributes. It also defines a nice display method.
The resulting class is essentially the same as the Javascript object.

To better match the Python:

• change namespace.isset() to use _.has() instead of !! (!!null is false)
• remove the null test in namespace.set()
• the _.extend() alternative is not used.
• namespace.get() is not used