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_mailgun_webhooks.py

@@ -4,7 +4,7 @@ from datetime import datetime
 import hashlib
 import hmac
 from django.core.exceptions import ImproperlyConfigured
-from django.test import override_settings
+from django.test import override_settings, tag
 from django.utils.timezone import utc
 from mock import ANY
 
@@ -59,6 +59,7 @@ def querydict_to_postdict(qd):
     }
 
 
+@tag('mailgun')
 class MailgunWebhookSettingsTestCase(WebhookTestCase):
     def test_requires_api_key(self):
         with self.assertRaises(ImproperlyConfigured):
@@ -66,6 +67,7 @@ class MailgunWebhookSettingsTestCase(WebhookTestCase):
                              data=json.dumps(mailgun_sign_payload({'event-data': {'event': 'delivered'}})))
 
 
+@tag('mailgun')
 @override_settings(ANYMAIL_MAILGUN_API_KEY=TEST_API_KEY)
 class MailgunWebhookSecurityTestCase(WebhookTestCase, WebhookBasicAuthTestsMixin):
     should_warn_if_no_auth = False  # because we check webhook signature
@@ -94,6 +96,7 @@ class MailgunWebhookSecurityTestCase(WebhookTestCase, WebhookBasicAuthTestsMixin
         self.assertEqual(response.status_code, 400)
 
 
+@tag('mailgun')
 @override_settings(ANYMAIL_MAILGUN_API_KEY=TEST_API_KEY)
 class MailgunTestCase(WebhookTestCase):
     # Tests for Mailgun's new webhooks (announced 2018-06-29)
@@ -445,6 +448,7 @@ class MailgunTestCase(WebhookTestCase):
         self.assertEqual(event.click_url, "https://example.com/test")
 
 
+@tag('mailgun')
 @override_settings(ANYMAIL_MAILGUN_API_KEY=TEST_API_KEY)
 class MailgunLegacyTestCase(WebhookTestCase):
     # Tests for Mailgun's "legacy" webhooks