turn deprecated slice methods into wrappers

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

Therefore turning these methods into wrappers of __delitem__() and
__setitem__() now, de-duplicates code and makes implementing negative
slice indices simpler.

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