Log warning and error messages are not printed to console

[djhoese]

Describe the bug
In python 3 doing logger.warning or logging.error should print to the console/terminal by default. However, in #146 we added a NullHandler to the logger if one was not already created and this is causing the warn/error messages to not be printed. We should update this to only add the NullHandler if the user is not in an interactive environment.

After testing a few things I've noticed that in:

• Python 2

  Not having a handler configured and using the logger produces No handlers could be found for logger "satpy.my_test" and no log messages.

• Python 3

  Not having a handler configured and using the logger causes warnings/errors to be printed out. Which IMO is desired.

To Reproduce

Use Satpy and cause any error/warning message to be logged.

Expected behavior
Print error/warning log messages if no handler is configured (default for python 3). No warning about a missing handler when no handler is configured.

Since we still support python 2, my proposed solution is to only add the NullHandler if we are on python 2 and we don't have any handlers configured.