Fahrenheit support

[petervizi]

Thanks to dirkleas

class Fahrenheit(Unit):
"""
Degree Fahrenheit unit class.
"""

def __init__(self):
    """
    Initialize the `Unit` parameters with Fahrenheit.
    """
    Unit.__init__(self, 'Fahrenheit', 'derivedSI', '°F')

[petervizi]

Commited in 33845ca