Implement respawn API.

see (https://github.com/SpigotMC/Spigot-API/pull/9)
Tested, works.

Example autorespawn plugin:
```java
    @EventHandler(priority = EventPriority.HIGHEST)
    public void onDeath(final PlayerDeathEvent ev) {
        Bukkit.getScheduler().scheduleSyncDelayedTask(plugin, new Runnable() {
            public void run() {
                ev.getEntity().respawn();
            }
        });
    }
```