Add support for CLIENT commands

This pull request adds support for `CLIENT [LIST | KILL | GETNAME | SETNAME]` commands. 

I think that some of tests might be able to be improved. For example, the current test for `CLIENT KILL` requires Redis 2.6.9 or later because the test depends on `CLIENT SETNAME`. However, the test should not use such newer commands and run on Redis 2.4.0 or later. If you have any ideas for the improvement of tests, please let me know.