Module PluginManager

The basic interface and implementation for a plugin manager.

Also define the basic mechanism to add functionalities to the base PluginManager. A few "principles" to follow in this case:

If the new functionalities do not overlap the ones already implemented, then they must be implemented as a Decorator class of the base plugin. This should be done by inheriting the ``PluginManagerDecorator``.

If this previous way is not possible, then the functionalities should be added as a subclass of ``PluginManager``.

The first method is highly prefered since it makes it possible to have a more flexible design where one can pick several functionalities and litterally *add* them to get an object corresponding to one's precise needs.