[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.
i wanted to do this when i first started developing blaze and @mrocklin suggested adding the top-level `blaze` directory to my `PYTHONPATH`. this has worked out extremely well as it enables me to locally set up `conda` environments and basically run tests across all python versions that we support without having to install anything. the downside is that this doesn't work with development on packages that include extension modules such as `pandas`.