fix on macOS when value is NoneType when compute the max values

Add a unit test for plugin's sorted stats method

Avoid Glances to crash with a IOError if duration <= 0 in the stdout display module (see https://stackoverflow.com/questions/36226054/strange-ioerror-when-time-sleep-in-python)

Use alias aware sorting also for network devices

Implement number and alias aware diskio sorting

The __cmp__() special method is gone in Python 3 in favor of rich
comparison methods.