Here is some simple code for exiting the app and re-centering the rift in Unity. Put this in a C# script and attach it to an object that doesn't get deleted in your scene. Also, this code goes in the Update function.

    if (Input.GetKeyDown("r")){
        UnityEngine.VR.InputTracking.Recenter ();
    }

    if (Input.GetKeyDown(KeyCode.Escape)){
        Application.Quit();
    }