Add support for handling references to None

This commit contains a small fix that allows the builder method to return None
and thus create a persistent None reference. A test is included that
illustrates this behaviour.
Implement support for assigning None

Now `None` can be assigned to a weak_and_lazy reference. However, this requires the
builder to support default arguments so that it can be called with no
arguments.
Merge pull request #1 from aepsil0n/topic/noneref

Support for None references