Replacing '== None' comparison with 'is None' (see PEP8)

http://www.python.org/dev/peps/pep-0008/#programming-recommendations

Comparisons to singletons like None should always be done with
is or is not, never the equality operators.

Also, beware of writing if x when you really mean if x is not None
e.g. when testing whether a variable or argument that defaults to None
 was set to some other value. The other value might have a type
(such as a container) that could be false in a boolean context!