Add PDF book to prepare OCPJP6 Java Certification

Was Escala, now Scala

Added PlugIn Tapestry book, a book in spanish about the framework Apache Tapestry for programming web applications in Java.

libro sobre Debian GNU/Linux para administradores

The link is outdated because the GitHub organization maintaining the book
changed their name. The link currently works because of a redirect, but it
would better to replace it with the current link.

This book is a spanish translation of the official guide "Getting Started
with Rails."

Data Visualization for the web.