Made the artifact 'kind' attribute optional.
Added artifact default image.
Started writing artifacts test.
Fixed ForeignKey calls. #64.
Merge branch 'master' into david-issue54
Saving before merging in most recent master version.
Merge branch 'master' into david-issue54

Conflicts:
	hogwarts/library/tests.py
Merge branch 'master' into david-issue54

Conflicts:
	hogwarts/library/models.py
Added more to Artifacts test.
Added and passed Book unit tests. #54.
Merge pull request #69 from Kae7in/david-issue54

Artifact and Book Model Unit Tests. Resolves #54.