Add Log Part.

[Zrealshadow]

I choose logback to implement the interface of slf4j.

• add dependency in pom.xml under root directory.
• add configuration for log in test part, which can control all logger in class under cool-core. However, I only recommend add logger in test class. For classes under main, add logger will affect efficiency, which is not ideal for system. I recommend to add logger for unittest or server code. If classes outsize of cool-core, add another configuration file plz.
• In logback-test.xml configuration file, I add File and stderr Appender, which will print msg in terminal and record msg in log file named cool.log under cool-core directory.

Other modification

• Add some necessary msg through logger in unittest
• Delete print code in main code.

[Zrealshadow]

For the commit log, I forget to merge first and create branch later. I think if we merge, the duplicated commit log will be deleted. Right ?

[Zrealshadow]

Are we going to use it to replace all the System.out.println for other internal logging?

Also, if we get it for internal logging, we shall test the added dependency compatibility with those introduced by Naili,

Let's merge this after Naili's distributed PR.

Actually, the mechanism of java log pattern is that we have to create a logger instance as a attribute in every class if you want to trace the log. In my opinion, it will cost a lot if we add this mechanism in cool-core part. Obviously for a system which emphasizes efficiency, it is not a good idea.

If we must add log in cool-core part to moniter the process of this system, I have a proposal. We create a singleton class, in which we staticaly initialize a log pattern. Whenever we have to get record in cool-core, we call this static singleton class. The shortcut of this is that we can not trace the log accurately, but it will save much memory usage considering the lots of class.