Update SchemaABC

Also, update exception docstrings
Rename test modules
Flexibly inherit fields following method resolution order.

Allow schema subclasses to inherit fields from plain base classes.
Inherit fields according to MRO rather than recursing over bases.
Add tests for above.
Small refactor to looping over mro
Update changelog and and @jmcarp to AUTHORS
Merge branch 'jmcarp-dev' into dev

Conflicts:
	tests/test_schema.py