Use tox-docker to run services when testing

[sajith]

Resolves #241. Changes:

• Use tox-docker to spin up/down MongoDB and RabbitMQ services when testing with tox. This is a little more convenient than having to do some manual setup before testing locally.
• README updates to reflect the above.
• CI updates to reflect the above. On CI, we will run pytest instead of tox, so that reported coverage does not drop. (Reason is this: some MQ code gets exercised when running pytest, although we do not have tests that use MQ directly. When running tox though, MQ_HOST is unset, and that will cause that part of code to not get invoked at all. I'm planning to add some tests that use MQ, so this is a temporary workaround.)

[coveralls]

Pull Request Test Coverage Report for Build 8159364932

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 38.63%

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Totals
Change from base Build 7916946928:	0.4%
Covered Lines:	619
Relevant Lines:	1442

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💛 - Coveralls