Add WebSocketAutoConfiguration

Opinionated defaults for WebSockets:

* If spring-websocket is on the classpath and so is
the Tomcat WSci initializer then it is added to the context
* A DefaultSockJsService is added if none is present
* User has only to define @Beans of type WebSocketHandler with
name starting "/"
* Each one is converted to a SockJsHttpRequestHandler and
mapped to "/<beanName>/**"