[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.
you can collapse this and the dispatch on `Not` above into a single function:

```python
@dispatch((USub, Not), SciDBArray)
def compute_up(t, x, **kwargs):
    return t.op(x)
```