Added null() support

[eskuat]

Fix for Issue #55

[edelooff]

Instead of having the test fail when either of these fail, parametrize over the different null-like values:

Suggested change

	def test_null_none(session):
	"""
	Make sure library can handle both None and null() as JSON value
	"""
	for value in (None, null()):
	author = Author(name="John Doe", handles=value)
	Article(author=author.name, content="very important", references=value)
	@pytest.mark.parametrize("null_like", [None, null()])
	def test_null_like_assignments(null_like):
	"""Make sure library can handle both None and null() as JSON value."""
	author = Author(name="John Doe", handles=null_like)
	assert author.handles is None

[edelooff]

Apologies for the delay. The change looks good but the testing needs a small change to separate the two actual tests.