Testing: Generate queries and compare results against other databases

This is the intital commit and is a work in progress. See the README for a
list of possible improvements.

As an overview of how the files are related:

  model.py: This is the base upon which the other files are built. It
      contains something like a grammer for queries.

  query_generator.py: Generates random permutations of the model.

  model_translator.py: Produces SQL based on the model

  discrepancy_searcher.py: Uses the above to generate, run, and compare
      query results.

Change-Id: Iaca6277766f5a86568eaa3f05b99c832942ab38b
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1648
Reviewed-by: Casey Ching <casey@cloudera.com>
Tested-by: Casey Ching <casey@cloudera.com>