Add module `test_models`.

The responsibility of this new module is to test all virtual model attributes.
As is, the new module only tests a single method on a single class. However,
this small amount of code provides a template for writing many future tests.
Test method models.Character.__str__

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.fatigue

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.hitpoints

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.alertness

Add a test for the aforementioned method to module `test_models`.
Test method models.Character.will

Add a test for the aforementioned method to module `test_models`. In the process
of testing this method, fix an error in class `Character`.
Test method models.Character.fright

Add a test in module `test_models`.
Test method models.Character.initiative

Add a test in module `test_models`.
Test method models.Character.no_encumbrance

Add a test in module `test_models`. Fix a typo in the method name.
Test encumbrance-related model.Character methods.

Add tests for the following methods:
* light_encumbrance
* medium_encumbrance
* heavy_encumbrance
* extra_heavy_encumbrance

Also fix several spelling errors. Change the word "encumberance" to
"encumbrance".
Simplify random selection of element from list.

random.choice([...]) is equivalent to random.select([...], 1)[0]. Use the
former, as it is simpler.
Add factories for Skill and SkillSet models.
Test methods on Skill and SkillSet models
Fix up some minor issues.

* Trim down and move some comments.
* Expand doctest for method factories.character_floatfield.
* Fix a bug in method factories.skill_name.
Add class TraitFactory and supporting methods.
Add tests for Trait model.
Add class ItemFactory and helper methods
Add tests for Item model
Add class PossessionFactory and helper methods
Add class CharacterSkillFactory and helper methods