Simplify using Pippo with externaly created servers

[gkresic]

Few observations:

pippo-demo-servlet demonstrates how Pippo could currently be embedded into any servlet container. Provided way works, but doesn't play well with DI concepts. It even, as far as I can see, creates two instances of ServletApplication: one in ServletDemo.java:27 and another one in PippoFilter.java:259

PippoServletContextListener is another way, but it requires user to produce its own server initializer. However, there is no docs on what is needed to configure the server (just registering PippoFilter?) nor default implementation exists (at least not for Jetty).

If user opts to use provided WebServers (JettyServer, TomcatServer and UndertowServer) it is limited to either use internal, Pippo-created server or extend any of those classes, but loose ServiceDiscovery bindings.

Finally, when using external server, Pippo class is not needed at all. If fact, since it represents just a bundling of Application and (internally managed) WebServer, it may as well be renamed to PippoLauncher.