action with assync result

Current implementation of creating actions is blocking: 

```
def action[T](name: String)(action: T => ActionResult[T]): ActionField[T]
```

 "save" or "delete" actions could potentially communicate with DB so there'll be nice if they could use async API. This could be better approach for some usecases:  

```
def action[T](name: String)(action: T => Future[ActionResult[T]]): ActionField[T]
```
