bug #74 Default font size should be

The « getter » of fontSize was not called, because the wrong attribute
(._book) was calling it. We added a new attribute (._bookViewer) to the
object ChapterContents, so then it can call the right function.
This call to `console.log` doesn't match the others. Either remove this call to `console.log` or make it look like
````js
console.log("BookViewer", "constructor", "fontSize", this._fontSize);
````