comprehensive compute tests

Sets up a suite of comprehensive tests against a variety of backends.  Fixes various errors that were exposed.

This PR can be most readily understood by looking at the source of https://github.com/mrocklin/blaze/blob/compute-comprehensive/blaze/compute/tests/test_comprehensive.py which is relatively simple.
Yes, it would be reasonable.  I guess I opt for importing unqualified terms if I'm going to use them a lot and if it's unambiguous what they are.  If there were other projects that used the terms DataFrame and Series in our domain then I would feel more compelled to add the `pd` term as we do for `nd.array` and `np.array`.  In this case I don't see much added value in adding the `pd.` namespace qualifier.  Happy to do it throughout Blaze though if you think it's a good practice.