Negative indexing in distarrays.

I allow distarrays to take negative indices by converting the negative indices to positive ones in `__getitem__` and `__setitem__`. This seems like the easiest way but it is probably not the best. 

I originally tried altering the Map classes to take negative indices but there were other checks happening that were throwing index errors. So that way seemed pretty invasive.

Also, look at the tests. Are the places I added tests the only cases that we are currently checking indexing directly?