Added ability to drag/drop new entries in tabular inline sets.

In order to implement this, I needed to make two changes, one of
which is fairly significant.

1) The javascript no longer limits the drag mechanism to <td>'s with
the "has_original" class.
2) The CustomInlineFormSet.save_new() function now checks default_sort_order
to see whether it's 0, intead of checking its type. This necessitated a change
to the documentation, stating that "default=0" must be added to the sort field.

The default has to be zero for two reasons:

1) It simplifies that check in CustomInlineFormSet.save_new(). A default of 0 is
compatible with the javascript, since it uses 1-indexed sorting.
2) Users of the newest South versions are forced to provide a default value when
they add a new sort field to their model.

Ultimately, this change allows us to know for certain what the default value will
be for a new inline entry which has not yet been dragged, allowing the max_order
algorithm to work properly in all cases.
Fixed the way that the CustomInlineFormset class gets used.

Rather than executing inlineformset_factory() in the constructor,
SortableInlineAdminMixin now specifies the 'formset' class attribute just
like admin.InlineModelAdmin does it. Since users already have to put the
SortableInlineAdminMixin before admin.TabularInline in the class definition,
this means that admin.InlineModelAdmin.formset gets overridden, letting
admin.InlineModelAdmin.get_formset() do it's job as usual.
Merge pull request #11 from coredumperror/master

Added ability to drag/drop new entries in tabular inline sets.