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_backend.py

@@ -16,8 +16,7 @@ from email.mime.image import MIMEImage
 
 from django.core import mail
 from django.core.exceptions import ImproperlyConfigured
-from django.test import SimpleTestCase
-from django.test.utils import override_settings
+from django.test import SimpleTestCase, override_settings, tag
 from django.utils.timezone import get_fixed_timezone, override as override_current_timezone
 
 from anymail.exceptions import (
@@ -30,6 +29,7 @@ from .utils import (AnymailTestMixin, sample_email_content,
                     sample_image_content, sample_image_path, SAMPLE_IMAGE_FILENAME)
 
 
+@tag('mailgun')
 @override_settings(EMAIL_BACKEND='anymail.backends.mailgun.EmailBackend',
                    ANYMAIL={'MAILGUN_API_KEY': 'test_api_key'})
 class MailgunBackendMockAPITestCase(RequestsBackendMockAPITestCase):
@@ -44,6 +44,7 @@ class MailgunBackendMockAPITestCase(RequestsBackendMockAPITestCase):
         self.message = mail.EmailMultiAlternatives('Subject', 'Text Body', 'from@example.com', ['to@example.com'])
 
 
+@tag('mailgun')
 class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
     """Test backend support for Django standard email features"""
 
@@ -374,6 +375,7 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         self.assertEqual(sent, 0)
 
 
+@tag('mailgun')
 class MailgunBackendAnymailFeatureTests(MailgunBackendMockAPITestCase):
     """Test backend support for Anymail added features"""
 
@@ -563,6 +565,7 @@ class MailgunBackendAnymailFeatureTests(MailgunBackendMockAPITestCase):
     # (Anything that requests can serialize as a form field will work with Mailgun)
 
 
+@tag('mailgun')
 class MailgunBackendRecipientsRefusedTests(MailgunBackendMockAPITestCase):
     """Should raise AnymailRecipientsRefused when *all* recipients are rejected or invalid"""
 
@@ -594,11 +597,13 @@ class MailgunBackendRecipientsRefusedTests(MailgunBackendMockAPITestCase):
         self.assertEqual(sent, 0)
 
 
+@tag('mailgun')
 class MailgunBackendSessionSharingTestCase(SessionSharingTestCasesMixin, MailgunBackendMockAPITestCase):
     """Requests session sharing tests"""
     pass  # tests are defined in the mixin
 
 
+@tag('mailgun')
 @override_settings(EMAIL_BACKEND="anymail.backends.mailgun.EmailBackend")
 class MailgunBackendImproperlyConfiguredTests(SimpleTestCase, AnymailTestMixin):
     """Test ESP backend without required settings in place"""