simplify Application creation; update url dispatch guide section

examples/basic.rs

@@ -57,7 +57,7 @@ fn main() {
     let sys = actix::System::new("ws-example");
 
     HttpServer::new(
-        Application::new("/")
+        Application::new()
             // enable logger
             .middleware(middlewares::Logger::default())
             // cookie session middleware

examples/state.rs

@@ -60,7 +60,7 @@ fn main() {
     let sys = actix::System::new("ws-example");
 
     HttpServer::new(
-        Application::with_state("/", AppState{counter: Cell::new(0)})
+        Application::with_state(AppState{counter: Cell::new(0)})
             // enable logger
             .middleware(middlewares::Logger::default())
             // websocket route

examples/websocket.rs

@@ -61,7 +61,7 @@ fn main() {
     let sys = actix::System::new("ws-example");
 
     HttpServer::new(
-        Application::new("/")
+        Application::new()
             // enable logger
             .middleware(middlewares::Logger::default())
             // websocket route