Add shutdown hook

[boyter]

Should add a shutdown hook allowing as clean as possible termination of the application. Java 8 makes this pretty easy,

Runtime.getRuntime().addShutdownHook(new Thread(() -> {
     // Do stuff in here
 }));

Example, http://stackoverflow.com/questions/37696588/java-8-and-lambdas-in-shutdown-hook#37696655

Need to implement https://github.com/boyter/searchcode-server/issues/41 first though.