Refs #113 - Enable customization of comparator field name in RowExists and RowsExist validation constraints.
Refs #113 - Create initial RowExists validator test.
Refs #113 - Test that violation added if entity not found.
Refs #113 - Test that custom fields are used for mapper search.
Refs #113 - Test rows exist validator.
Merge pull request #113 from synapsestudios/feature/113-modify-rows-exists

Modify Validation Constraint RowExists