Minor aesthetic tweak in the composer.json file
Removed a TODO from the processConfigurationValues() method.

The original TODO has been transformed into the issue #121 of the project.
Removed the validateConfiguration() method.

It's "impossible" to validate the book configuration because
the author can really set anything in each of the book fields
and he/she can even define any number of new fields.

In the future, it coud be possible to restrict some of the most
important fields, such as the title, the author/s name/s and the
publication date.
The user should not be able to define the slug programatically.
He/she can always modify the generated slug by renaming the
book's directory.
Removed another TODO from the code (according to the suggestions
of SensioLabs Insight).

This TODO is actually stored as the issue #59 of the project.
Removed a leftover hack introduced before the theme refactoring.
Removed another TODO from the code.

This TODO has been transformed into the issue #122 of the project.
Removed two more outdated and unnecesary TODO from the code.
Removed another TODO from the code and transformed it into the
issue #123 of the project.
Removed a TODO and transformed it into the issue #124 of the project
Removed a bunch of TODO related to the naming of the generated
book file. These TODO has been transformed into the issue #125
of the project.