Cleanup for entities from last commit.

Also added specs for `InstanceVariableSetter` class.

RSpec: 851 examples, 0 failures (but see Issue #87)
RuboCop: 162 files inspected, no offenses detected
New entity-factory classes and specs.

This concludes Step 3 of The Plan; see Issue #111.

RSpec: 855 examples, 0 failures (but see Issue #87)
RuboCop: 166 files inspected, no offenses detected