Configuration and plugin loading updates

* Config file section `rooms` is now `dict` instead of `list of dicts`
* Config file section `plugins` is now `dict of dicts` instead of `lis of dicts` and plugin name is used as a key for internal dicts.
* Improve plugin template: less code, using built-in `self.__class__.__name__`
* Improve HiPlugin example: less code, remove unused overloads. 
* Now handling `ImportError` on plugin import, when plugin is described in configuration file, but doesn't exist in Plugin directory