Negative indices don't work in __getitem__ / __setitem__ ?

[bgrant]

[kwmsmith]

@markkness after #247 is merged, we can start working on this one.

[kwmsmith]

Moving to next milestone.

[kwmsmith]

@cowlicks -- Indexing should use the Distribution object as much as possible. I'd like all the indexing logic to be there, not in DistArray.

[cowlicks]

Closed by #338