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

tests/test_sparkpost_integration.py

@@ -2,18 +2,17 @@ import os
 import unittest
 from datetime import datetime, timedelta
 
-from django.test import SimpleTestCase
-from django.test.utils import override_settings
+from django.test import SimpleTestCase, override_settings, tag
 
 from anymail.exceptions import AnymailAPIError
 from anymail.message import AnymailMessage
 
-from .utils import AnymailTestMixin, sample_image_path, RUN_LIVE_TESTS
+from .utils import AnymailTestMixin, sample_image_path
 
 SPARKPOST_TEST_API_KEY = os.getenv('SPARKPOST_TEST_API_KEY')
 
 
-@unittest.skipUnless(RUN_LIVE_TESTS, "RUN_LIVE_TESTS disabled in this environment")
+@tag('sparkpost', 'live')
 @unittest.skipUnless(SPARKPOST_TEST_API_KEY,
                      "Set SPARKPOST_TEST_API_KEY environment variable "
                      "to run SparkPost integration tests")