Disclaimer: this is to demo RESTful Web Service

1, Embeded tomcat and in-memory database hsqldb are used.
2, java version is 1.8
3, I) run as a standalone application by
        a). build the project, using "mvn clean install"
        b). run by "java -jar path/Sample-1.0-SNAPSHOT.jar"
        cI. stop by "kill -9 <pid>". get pid by "jps -lv |grep Sample"
   II) run the source code (Application class as entry point) in an IDE,
       intellij for instance.
4, test code is omitted as the scenario is simple.
5, for Part 1, duplicate user are not to be saved.
   for Part 2, empty keyword not allowed as per requirement.