[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.
Suggest something like 

```Python
exprs = [t.amount.mean(), t.amount.count(), t.amount.sum(), ...]
for expr in exprs:
    assert (compute(expr, x) == compute(expr, x_np)).all()
```