Revert "Début de la gestion de la persistence"

This reverts commit f89fad0578192b516840dba004edae39144820ce.
Support persistence through Objectify

pom.xml: Add Objectify dependency

src/main/java/fr/univnantes/atal/poubatal/datastore/User.java: Add annotations and complete load() + save().

src/main/webapp/WEB-INF/web.xml: Add Objectify configuration.

src/main/java/fr/univnantes/atal/poubatal/datastore/OfyService.java: Project's version of ofy() to register project's classes.fix

src/main/webapp/poubatal.jsp: Sample test with persistence.