rename all "tutorial" with "Essentials"
Merge branch 'master' into chapter9
* rename all "tutorial" with "Essentials"

Conflicts:
	pom.xml
	src/main/java/org/zkoss/essentials/chapter4/SidebarPageConfigChapter4Impl.java
	src/main/java/org/zkoss/essentials/chapter5/AuthenticationServiceChapter5Impl.java
	src/main/java/org/zkoss/essentials/chapter5/UserInfoServiceChapter5Impl.java
	src/main/java/org/zkoss/essentials/chapter6/TodoListServiceChapter6Impl.java
	src/main/java/org/zkoss/essentials/chapter8/AuthenticationServiceChapter8Impl.java
	src/main/webapp/index.zul