turn deprecated slice methods into wrappers

__delslice__() and __setslice__() are deprecated as of python v2.6 [1]
and urwid does not support python versions  pre-v2.6 (anymore).

Making these methods wrappers of __delitem__() and __setitem__()
de-duplicates code (and doctest's)

[1] https://docs.python.org/2.6/reference/datamodel.html
add doctest examples of negative indices
make __delitem__ support negative indices