[WIP] SciDB backend

This isn't ready to merge, but I've started playing around with connecting SciDB to blaze.

There are two failing tests currently (I think one is a backend bug, and one is a SciDB bug). I also want to add some more tests to make sure I understand how blaze is working.
Sorry, should have said `isnull`.  I was speaking within the limited context of NumPy, in which these concepts aren't quite as clean.