Removed strange import

Removed the "from .datasource_plugin import DatasourcePlugin" in
Plugin.register

lodel/plugin/plugins.py

     #@throw PluginError
     @classmethod
     def register(cls, plugin_name):
-        from .datasource_plugin import DatasourcePlugin
         if plugin_name in cls._plugin_instances:
             msg = "Plugin allready registered with same name %s"
             msg %= plugin_name