Add example with usual system (#142)

* Add example with usual system

* Only build with parallel feature

* Reformatting and add actual entities

* Revert formatting in other examples

examples/sys.rs

@@ -0,0 +1,63 @@
+extern crate specs;
+
+use specs::{Component, VecStorage, World};
+
+struct Vel(f32);
+struct Pos(f32);
+
+impl Component for Vel {
+    type Storage = VecStorage<Vel>;
+}
+
+impl Component for Pos {
+    type Storage = VecStorage<Pos>;
+}
+
+#[cfg(not(feature="parallel"))]
+fn main() {}
+
+#[cfg(feature="parallel")]
+fn main() {
+    use specs::{RunArg, Planner, System};
+
+    let mut planner = {
+        let mut world = World::new();
+        world.register::<Pos>();
+        world.register::<Vel>();
+
+        world
+            .create_now()
+            .with(Vel(2.0))
+            .with(Pos(0.0))
+            .build();
+        world
+            .create_now()
+            .with(Vel(4.0))
+            .with(Pos(1.6))
+            .build();
+        world
+            .create_now()
+            .with(Vel(1.5))
+            .with(Pos(5.4))
+            .build();
+
+        Planner::new(world)
+    };
+
+    struct SysA;
+
+    impl System<()> for SysA {
+        fn run(&mut self, arg: RunArg, _: ()) {
+            use specs::{Gate, Join};
+
+            let (pos, vel) = arg.fetch(|w| (w.write::<Pos>(), w.read::<Vel>()));
+
+            for (pos, vel) in (&mut pos.pass(), &vel.pass()).join() {
+                pos.0 += vel.0;
+            }
+        }
+    }
+
+    planner.add_system(SysA, "a", 1);
+    planner.dispatch(());
+}