sbox check and update runtime

[ChanDaoH]

sbox 启动逻辑：

1. 如果本地不存在 runtime 镜像，则直接 docker run
2. 如果本地存在 runtime 镜像，将和远端最新的 runtime 镜像做比对，如果不相同则提示用户更新

   $ fcli shell
   Welcome to the function compute world. Have fun!
   >>> sbox -d . -t nodejs6
   Warning: Your aliyunfc/runtime-nodejs6:build image is not the latest version
   Warning: You can use 'docker pull aliyunfc/runtime-nodejs6:build' to update image
   Entering the container. Your code is in the /code direcotry.
   root@xxxxx:/code# 

[vangie]

Warning: Your aliyunfc/runtime-nodejs6:build image is not the latest version Warning: You can use 'docker pull aliyunfc/runtime-nodejs6:build' to update image

Warning 和正文之间能加一个空行么，这样不太容易被忽略。

[vangie]

代码没有测试