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
Add failing test for checking required on load
Merge branch 'dev' into load-required
Validate required fields upon deserialization

see #45
Handle unicode output during validation
Python 2 unicode compatibility in _WrappingException
Exclude missing values from deserialization output
Add ValidationError that gets wrapped by UnmarshallingError
Merge branch 'amikholap-unicode_output_for_validators' into dev
Add @amikholap to AUTHORS
Move validation tests from test_serialization -> test_deserialization
Don't validate required fields upon serialization
Move validation to its own method
Skip validation on serialization

See comments in #45 and #47
Update docs