JSONSchema().dump(schema) does not support allow_none in schema

[NicholasHyland]

If I have the following:

class mySchema(Schema): id = Int(allow_none=True)

JSONSchema().dump(mySchema) will return 'id': {'title': 'id', 'type': 'number', 'format': 'integer'} but will not specify that it can be None. As a result, when it is None, I get a None is not of type 'number' error.

[avilaton]

This is fixed in #106 and should be closed.