Add websockets support to EmbeddedJetty.

Usage:
````
EmbeddedJetty server = new EmbeddedJetty();
server.addWebSocketEndpoint(MyEndpoint.class) // MyEndpoint is a class annotated with @ServerEndpoint
server.start();
````
Merge pull request #28 from troggy/feature/jetty-websockets

Add websockets support to EmbeddedJetty.