add basic user entity/document and user repository for object-relational mapping

src/main/java/com/cloudcomputing/todo/entity/User.java

+package com.cloudcomputing.todo.entity;
+
+import org.springframework.data.annotation.Id;
+import org.springframework.data.mongodb.core.mapping.Document;
+
+@Document(collection = "users")
+public class User {
+    @Id
+    private Long user_id;
+    private String user_name;
+    private String password;
+}

src/main/java/com/cloudcomputing/todo/repository/UserRepository.java

+package com.cloudcomputing.todo.repository;
+
+import com.cloudcomputing.todo.entity.User;
+import org.springframework.data.mongodb.repository.MongoRepository;
+
+public interface UserRepository extends MongoRepository<User, String> {
+}