Extent rule NUnit2010 to detect 'is null'

[manfred-brands]

Rule NUnit2010 fires for: Assert.That(instance == null) but not for Assert.That(instance is null).
It should also detected is not null to mean != null.
Code fixes should create: Assert.That(instance, Is.Null) and Assert.That(instance, Is.Not.Null) respectively.

[manfred-brands]

PR can also deal with more complex is pattern expressions like:
is < 0 or >= 1
which gets translated into
Is.LessThan(0).Or.GreaterThanOrEqualTo(1)