Rename test_serialization -> test_marshmallow
Initial implementation of two-way nesting

Adds a registry of serializer classes
[see #19]
Raise RegistryError if class name is invalid
Update docs
Keep all classes with a given name
Prevent error when a module is reloaded
Fix test commands
Add test watching command
If 2 serializers have the same name, can use full module path
Update docs