Skeleton for remote query execution using celery.

[bkyryliuk]

Current change adds ability to execute the SQL queries on the celery worker.
More info about celery can be found here:

• http://docs.celeryproject.org/en/latest/getting-started/first-steps-with-celery.html

Worker can be started via command caravel worker
Celery is configured in the config.py and by default it is turned off. (CELERY_CONFIG = None)
Fixed some lint issues in the tests.

Tested:

1. Manually tested if the query workes fine with the CELERY_CONFIG = None and with

class CeleryConfig(object):
    BROKER_URL = 'sqla+sqlite:///celerydb.sqlite'
    CELERY_IMPORTS = ('caravel.tasks', )
    CELERY_RESULT_BACKEND = 'db+sqlite:///celery_results.sqlite'
    CELERY_ANNOTATIONS = {'tasks.add': {'rate_limit': '10/s'}}
CELERY_CONFIG = CeleryConfig

Task is passed and executed:

Query works for both cases:

Reviewers:

• @mistercrunch

[coveralls]

Coverage decreased (-0.008%) to 80.816% when pulling fc94502 on bkyryliuk:celery into 1a43349 on airbnb:carapal.

[bkyryliuk]

pushed commit 9fc0e84 to handle the 'undefined' schema case.

Tested the mysql queries. Will manually test the presto ones and if the schema functionality works well will my implementation.

[coveralls]

Coverage decreased (-0.008%) to 80.816% when pulling 9fc0e84 on bkyryliuk:celery into 1a43349 on airbnb:carapal.