Improve test coverage.
Start of field fallback implementation.
Fallback field build_value implementation.
Support fallbacks in fields.
Merge branch 'develop' into feature/issue-3-fallback-field-access
String formatting fallback helper.
Optional fallback fields should be optional.
required_fallback should default to True.
Rearrange config field setup a bit.
Rework fallbacks a bit.
Rewrite field fallbacks again with a nicer API.
Exclude _version.py from coverage and configure branch checking.
Remove AssertionError coverage exclusion.
Merge branch 'feature/issue-3-fallback-field-access' into develop
Merge branch 'feature/adhoc-better-coverage-config' into develop
Merge branch 'develop' into feature/use-tox

Conflicts:
	.coveragerc