Spark Java Article BAEL-498 (#912)

* Initial Commit for Spark Java Article BAEL-498

* reverting main pom.xml and rollbacking accidental changes.

* Changes as per review:

1. Added UserService
2. Renamed UserStore to UserServiceMapImpl
3. Removed Empty spaces in User.java
4. Removed AppTest
5. Changes in SparkRestExample for using UserServiceMapImp instead of
UserStore static functions.

* Suggested changes in print messages.

* Changes as per comments on github... for PR:
https://github.com/eugenp/tutorials/pull/912

* Changes in editUser function as per guidance by Kevin.

* Clean up

* added 1.8 config for pom.xml

* Clean up.

* Removed junit dep.

* Added Application/json in response type.

spark-java/src/main/java/com/baeldung/sparkjava/UserService.java

@@ -0,0 +1,12 @@
+package com.baeldung.sparkjava;
+
+import java.util.Collection;
+
+public interface UserService {
+    public void addUser (User user) ;
+    public Collection<User> getUsers () ;
+    public User getUser (String id) ;
+    public User editUser (User user) throws UserException;
+    public void deleteUser (String id) ;
+    public boolean userExist (String id);
+}