trivial fix debug information for all plugins

Now all plugins debug info will display like below:  

```
DEBUG:root:All plugins: [<yapsy.PluginInfo.PluginInfo object at 0x104eae310>, <yapsy.PluginInfo.PluginInfo object at 0x104d0ead0>, <yapsy.PluginInfo.PluginInfo object at 0x104d0e8d0>]
```
I think it is a little unreadable. So I modify this one line, and result will be like:  

```
DEBUG:root:All plugins: ChatRoom, Webserver, VersionChecker
```
I know the better way is to add `__repr__` method in the Plugin class to show list string, but it needs to be modified in **YAPSY** lib.