Using Gtk.Notebook on Controller to switch between views.
* Added controller to main window, istead of using WeekView directly.
* There is a bug on the signals being emitted (as in edit a task, for ex).
* The two weeks view is not working properly since a Week object has seven days.
Changes in the status bar.
* Removed label that was used as intermediator.
Fixed bug with multiple signals being emitted when changing views.
* Now the signals from the previous view are disconnected and then the new
  signals are reconnected.
Added current_view variable in CalendarPlugin to keep track of current view.
* This way it is not necessary to mimic every public function from a view
  inside the controller, we call the functions directly from the view object.
Modularized connection to current_view signals.
Changed Week to WeekSpan that shows N weeks instead of just one.
* Now two_weeks_view is working properly.
Fixed bug on size allocation of notebook.
* The view content wasn't expanding when enlarging window size.