[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.
can you call this as

```python
@pytest.mark.xfail(raises=ExceptionSubclass, reason='Short description of the reason')
```

the text in the `reason` parameter will show up when you run `py.test -r x` and let's us know why the `xfail`s are happening.