Make methods called by fields.Method classmethods
Make methods called by fields.Method classmethods

In our usage of this library, we have sorely needed to overload these
methods to make them work for us (i.e. required fields in nested
fields) so that's why they have been moved.

I also made the code less claustrophobic and let it breathe a little
bit.

fuhrysteve/marshmallow-jsonschema