Make skin loading asynchronously

[RecursiveG]

Current synchronous skin loading strategy causes severe game lag.

[xfl03]

When 'SkinManager' does 'loadSkin', it has been running in a thread named 'pool-x-thread-x'.
It isn't 'client-thread', so it's no use to make async loading again to avoid to affect game.
'SkinManager'调用'loadSkin'加载皮肤时，本已运行于一个独立于'client-thread'的名为'pool-x-thread-x'的线程中，感觉并不需要再次制作异步加载来避免对游戏主体产生影响。(为何不能好好说中文= =)
注：仅针对用户皮肤加载，头颅皮肤加载是主线程的任务

[RecursiveG]

Note:
SSP /give xx skull 1 3 {SkullOwner: oo}
CommandGive.java#L66
TileEntityItemStackRenderer.java#L74 stuck on client thread.

[xfl03]

Yes, I said that 'skull loads in client thread'.
But now, UniSkinMod has loading skull texture in a new thread, isn't it?