Merge pull request #963 from dingpinglv/Iss2269_FixBugForLabelTTF

Fixed #2269 fixed a bug of LabelTTF that setString is invalid when call setString("")
fixed #2269 Switch shader of cc.Sprite when call setTexture(null)
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-html5 into Iss2269_FixBugForLabelTTF