Add new sensors_fans method

[nicolargo]

Add a new method to grab the fans speed.

The method is called sensors_fans():

>>> import psutil
>>> psutil.sensors_fans()
defaultdict(<type 'list'>, {'dell_smm': [('Processor Fan', 0)]})

[giampaolo]

Thanks for this PR.
I'm gonna merge this but I will apply changes to the return value so that:

• we return a dict instead of a defaultdict
• dict value will be a list of namedutples

[giampaolo]

OK, here's the new API:

>>> import psutil
>>> psutil.sensors_fans()
{'asus': [sfan(label='cpu_fan', current=3200)]}

[giampaolo]

New 5.2.0 version is out on PYPI.