Adding a new test in LabelTest. Non-ascii UTF8 string from native should be converted to jsval correctly.

`LabelTest/LabelTTFChinese` can't work on cocos2d-iphone now. Nothing is shown in this test. I don't know why.
On Cocos2d-x jsb, it works ok now.
BTW, `cc.LabelTTF.string()` need to be `cc.LabelTTF.getString()`, `cc.LabelBMFont` as well.
So this Pull Request will break the JS Test in cocos2d-iphone jsb. 
Riq, could you please update cocos2d-iphone jsb to support this test?