Initial commit related to plug-in system:

- basic PluginManager class that loads plugins from *.py files
  in given directories
- Singleton metaclass was created to use with PluginManager;
  notice: __init__ of class is called only once (not like in code
	that is included in Python Cookbook)
- variable to keep paths of plugin directories has been created
  (common.gajim.PLUGINS_DIRS); also added initilization of these
  paths to common.ConfigPaths
- added global variable with PluginManager object:
  common.gajim.plugin_manager
- created customized logger for plugin system ('gajim.plugin_system')
- created function decorator plugins.helpers.log_calls which logs
  each call of function/method; it also logs when function is left
- base class Plugin for plug-in implementation added; not much
  here - only empty class attributes: name, short_name, authors,
	version, description
- based on Plugin class, first plugin was created named
  LengthNotifierPlugin; it is used to notify users when they
  exceed given length of message during writing it (text entry
  field highlights)
- first GUI extension points works when ChatControl object
  is created (it is used in mentioned plugin)
- added 'epydoc.conf' file customized a little bit (file
	is also in trunk now)
- fixed indentation in common.sleepy module (also in trunk
	now)