Pushed everythin into a class.

Having the configuration stored in a module variable makes it more difficult
to handle multiple configurations (from different modules).

By putting everything into a class, an API user may create as many instances
as he/she wants.
Some values are now instance vars & doc update
Moved the docstring to the class (for sphinx).
``Config`` now inherits from SafeConfigParser.

All the handling for unique instances has been removed. This sould be the
choice of the user of this library. Inheriting from ``SafeConfigParser`` gives
us all the expected functionality.
Better introspection on error.

* new member: active_path
* new member: loaded_files
Renamed `file_name` to `filename`.