Tox:
* Add Tox factor for extras (all, none, individual ESP).
For now, only break out ESPs that have specific extra
dependencies (amazon_ses, sparkpost).
* Install most package dependencies (including extras)
through the package itself.
* Use new runtests.py environment vars to limit test tags
when Tox isn't installing all extras.
Travis:
* Rework matrix to request specific TOXENVs directly;
drop tox-travis.
Test runner (runtests.py):
* Centralize RUN_LIVE_TESTS logic in runtests.py
* Add ANYMAIL_ONLY_TEST and ANYMAIL_SKIP_TESTS env vars
(comma-separated lists of tags)
Test implementations:
* Tag all ESP-specific tests with ESP
* Tag live tests with "live"
* Don't import ESP-specific packages at test module level.
(Test discovery imports test modules before tag-based filtering.)
Closes#104
Using undeliverable @example.com recipient addresses leads some ESPs
to flag the Anymail test accounts. Switch all live integration tests
to mailinator.com recipients (unless they were already using the ESP's
own "test sink" addresses).
