[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.
is this necessary? we may not want to default to converting to a `DataFrame` and then computing on that when an expression class isn't defined for the SciDB backend. we might want to just throw an error saying "hey that's not defined yet"