New features: custom base schema and NewType declarations

Fixes lovasoa#23
Fixes lovasoa#16
Fixes lovasoa#26

6.1.0rc2 Base Schema

Release Candidate: Add a NewType function

NewType allows creating custom types with custom
marshalling logic.

Switch to marshmallow 3

Adds support for Unions

Compatibility with marshmallow 3.0.0rc7

Add support for Any

Add support for the 'Any' type

Fix minor bug with NewType

Fix error with NewType