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.
This isn't something new from the patch, but would it be reasonable to do "import pandas as pd" and use pd.DataFrame/pd.Series later instead of importing them into the global namespace?