Fix None not being supported as Field values

[clarkduvall]

Fixes #56

This brings serpy more in line with DRF when encountering None values. For more info, see #56

The behavior for serpy is now:

	required=True	required=False
data = {'value': 1}	{'value': 1}	{'value': 1}
data = {'value': None}	{'value': None}	{'value': None}
data = {}	KeyError('value',)	{}

[coveralls]

Coverage decreased (-3.2%) to 96.774% when pulling f645934 on fix-none into 8954cdf on master.