This is related to #5

Basically, Whenever I add some validation like

import pandas schema as ps
# mumble mumble
ps.Column('some_column', [ps.validation.InListValidation(["FOO", "BAR", "BAZ", "nan"])]),

I have to add the "nan" to the list to avoid having null rows being assumed non valid, considering that some rows might contain null values or None. I understand that one might want to mark null values as invalid, so I propose different interface, something like

ps.Column('some_column', [ps.validation.InListValidation(["FOO", "BAR", "BAZ"])], ignore_nan=True),

This parameter ignore_nan can be set as False by default to get the current behaviour.