Fix NullHandler not allowing warning/error logs to be printed to console

[djhoese]

See #777 for details. This now results in python 3 printing the raw string of any warning or error messages from the logger and prints nothing with python 2 as the NullHandler is still added.

• Closes Log warning and error messages are not printed to console #777
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #778 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
+ Coverage   81.84%   81.84%   +<.01%     
==========================================
  Files         158      158              
  Lines       22735    22777      +42     
==========================================
+ Hits        18607    18642      +35     
- Misses       4128     4135       +7

Impacted Files	Coverage Δ
satpy/utils.py	62.96% <100%> (-0.47%)	⬇️
satpy/readers/avhrr_l1b_gaclac.py	90% <0%> (-1.43%)	⬇️
satpy/readers/__init__.py	93.67% <0%> (-1.21%)	⬇️
satpy/scene.py	90.59% <0%> (+0.5%)	⬆️

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[coveralls]

Coverage decreased (-0.03%) to 81.813% when pulling f9f583c on djhoese:bugfix-null-logger into 042426c on pytroll:master.

[coveralls]

Coverage decreased (-0.03%) to 81.813% when pulling f9f583c on djhoese:bugfix-null-logger into 042426c on pytroll:master.

[mraspaud]

LGTM