SORT_NONE_CLASS uses SORT_DESC_CLASS when specifying custom classes

[donbalajadia]

line 10 of templatetags/sortable.py should be:

SORT_NONE_CLASS = getattr(settings, 'SORT_NONE_CLASS' , 'sort-none')

instead of:

SORT_NONE_CLASS = getattr(settings, 'SORT_DESC_CLASS' , 'sort-none')