Refactor __getitem__; deprecate __getslice__
Refactor __getitem__; deprecate __getslice__

- Extend slicing to None, e.g. [:1], [1:], [:]
- Overload __getitem__ for negative indicies
- Remove __getslice__ only used in < Py 2.6
- Clarify docstrings
- Pass 2.7 nose

par2/lamana