错误信息：
Error: couldn't connect to server 127.0.0.1:27017 at src/mongo/shell/mongo.js:145
解决办法：
sudo rm /var/lib/mongodb/mongod.lock
sudo service mongodb restart
可以等一会以后在重复输入 service restart
http://stackoverflow.com/questions/19527564/mongo-couldnt-connect-to-server-127-0-0-127017-at-src-mongo-shell-mongo-js14