Chinese version of Scala tutorial for Java programmer

# Summary
1. Add the Flag of China image in PNG with size of 36 × 24: [resources/images/language/zh-cn.png](https://github.com/haoch/scala.github.com/blob/master/resources/images/language/zh-cn.png)
2. Translate [scala-for-java-programmer](https://github.com/haoch/scala.github.com/blob/master/tutorials/scala-for-java-programmers.md) into Chinese version : [Java程序员的Scala指南](https://github.com/haoch/scala.github.com/blob/master/zh-cn/tutorials/scala-for-java-programmers.md)
3. Add `zh-cn` language into label list of  [tutorials/scala-for-java-programmers.md](https://github.com/haoch/scala.github.com/blob/master/tutorials/scala-for-java-programmers.md)