Add Feature: Channel now can store or read setting values.

By adding two functions to Channel prototype: `set(key, value)` and `get(key)`, now Channel instance can store and read your custom setting values.

For example, as a kind of match game, we created a match room by `channelService.createChannel`.
The room instance needs to hold it's name, it's count of players, it's RoomMaster or it's password to deny players which don't have permission to come in.

So, I added this feature.

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对 Channel 的原型增加了如下两个方法, 这样你的 Channel 实例便可以存储或读取你可以自定义的值.
* `set(key, value)`
* `get(key)`

#### 为什么这样做呢

在某些对战类型的游戏中, 服务端通常使用`channelService.createChannel`来创建比赛房间.

而这个房间实例, 往往可能需要保存诸如: 房间名, 房间里的玩家数, 谁是房主, 或者是房主设置的密码, 以阻止不知道密码的玩家进入.