Changed all "== None" and "!= None" occurrences to comply to PEP8 and stop python 2.7 from giving FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
Merge pull request #89 from diegocortassa/DC-Patch001

Changed all "== None" and "!= None" to resp. "is None" and "is not None"