Support serializer inheritance

[nickswebsite]

Fields from base classes should be merged in with the subclassed serializer:

class BaseSerializer(Serializer):
    common_field_one = fields.StringField(name="commonFieldOne")

class ObjectToSerialize(object):
    def __init__(self):
        self.common_field_one = "Some Common Field One"
        self.specific_field = "Some Specific Field"

class DerivedSerializer(BaseSerializer):
    specific_field = fields.StringField(name="specificField")

serializer = DerivedSerializer(object=ObjectToSerialize())
serializer.validate()
serializer.data
{ "commonFieldOne": "Some Common Field One", "specificField": "Some Specific Field" }