Add missing HelloWorldServer mainClass referenced in pom

TL;DR:
 * HelloWorldServer and unit test copied and adapted from fluent-helloworld
 * Added mvn exec plugin configuration

Long version:
On second thought, I don't see why we should add fluent-helloworld as a dependency only for a one-liner, so I just copied it, added the adapted unit-test and the maven exec plugin configuration. 

I also closed [my first pull-request](https://github.com/CodeStory/fluent-blog/pull/1)