Fixed problem with split view not scrolling

As mentioned in Issue #4, the bottom window of the split view does not scroll in response to keyboard commands.

As it turns out this is because the view is added to a GtkScrolledWindow using add_with_viewport(). This is wrong. If the child element has native scrolling (which is the case here), it should be added with add().

This documentation ("http://www.pygtk.org/docs/pygtk/class-gtkscrolledwindow.html#method-gtkscrolledwindow--add-with-viewport") supports my conclusion, as does the fact that it resolves Issue #4... at least in Gedit 3.6.2.