Sparse-list

Thanks again, wetneb.

I had some issues with Twine & PyPi

This release addresses two issues raised by wetneb.

12

`del my_list[a:b]` now behaves like Python's builtin `list` class. (See 14)

13

`sparse_list[a:b]` now returns a `SparseList` itself. (See 15)

Thanks jaanauati!

It's been a long time coming but here's v0.6 with Python 3 and Extended Slicing Support.

Enjoy!