Change `== None` to `is None`

In this PR I did a replace for all files in `ginga`:
* `== None` -> `is None`
* `!= None` -> `is not None`

This should get rid of warnings and thus make real issues e.g. in #57 easier to spot.
This is also a PEP8 fix and as far as I know `is None` is always the preferred method to use (but I never bothered going through why that is).