Added dropwizard-embedded module, to embed a service in a daemon

This new module adds a new interface to a Dropwizard HTTP(s) service, so
that the HTTP server can be started/stopped at any point within an
already existing daemon process (no CLI required). This allows a
developer to embed Dropwizard into already existing services with their
own main run loop and SLF4J logging infrastructure.

Code example:
```java
public class WebService extends io.dropwizard.embedded.Service<Configuration> {
    public void initialize(EmbeddedBootstrap<Configuration> bootstrap) {
        // add bundles
        bootstrap.addBundle(new ViewBundle());
    }
    protected void start(Configuration config, Environment env) {
        // add resources
        env.jersey().register(new HelloWorldResource());
    }

    public static void main(String ... args) {
        handleCommandLine(args);
        configureSlf4jLogging();
        start();
        new MainRunLoop().run();
        stop();
    }
}
```
Can change of course - the reason for calling it start instead of run was to be explicitly different from `io.dropwizard.Application` and `io.dropwizard.cli.ServerCommand`, as those just have `run` while these have `start` and `stop`.