added tests for Tutorial class.
Made Tutorial#addChild and Tutorial#removeChild modify the child's parent (as well as the parent's children property)
test improvements: expect(X instanceof Array) --> Array.isArray(X), some toEqual -> toBe, .indexOf(X).not.toEqual(-1) --> .toContain(X)
Merge pull request #335 from mathematicalcoffee/tests-tutorial

added tests for Tutorial class.