Test without optional packages

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

Pipfile

@@ -14,12 +14,12 @@ six = "*"
 sparkpost = "*"
 
 [dev-packages]
-detox = "*"
+detox = "==0.18"
 flake8 = "*"
 mock = "*"
 sphinx = "*"
 sphinx-rtd-theme = "*"
-tox = "*"
+tox = "~=3.6.1"
 twine = "*"
 
 [requires]