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_mandrill_backend.py

@@ -7,8 +7,7 @@ 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 (AnymailAPIError, AnymailRecipientsRefused,
@@ -19,6 +18,7 @@ from .mock_requests_backend import RequestsBackendMockAPITestCase, SessionSharin
 from .utils import sample_image_content, sample_image_path, SAMPLE_IMAGE_FILENAME, AnymailTestMixin, decode_att
 
 
+@tag('mandrill')
 @override_settings(EMAIL_BACKEND='anymail.backends.mandrill.EmailBackend',
                    ANYMAIL={'MANDRILL_API_KEY': 'test_api_key'})
 class MandrillBackendMockAPITestCase(RequestsBackendMockAPITestCase):
@@ -35,6 +35,7 @@ class MandrillBackendMockAPITestCase(RequestsBackendMockAPITestCase):
         self.message = mail.EmailMultiAlternatives('Subject', 'Text Body', 'from@example.com', ['to@example.com'])
 
 
+@tag('mandrill')
 class MandrillBackendStandardEmailTests(MandrillBackendMockAPITestCase):
     """Test backend support for Django mail wrappers"""
 
@@ -267,6 +268,7 @@ class MandrillBackendStandardEmailTests(MandrillBackendMockAPITestCase):
             self.message.send()
 
 
+@tag('mandrill')
 class MandrillBackendAnymailFeatureTests(MandrillBackendMockAPITestCase):
     """Test backend support for Anymail added features"""
 
@@ -534,6 +536,7 @@ class MandrillBackendAnymailFeatureTests(MandrillBackendMockAPITestCase):
         self.assertRegex(str(err), r"Decimal.*is not JSON serializable")  # original message
 
 
+@tag('mandrill')
 class MandrillBackendRecipientsRefusedTests(MandrillBackendMockAPITestCase):
     """Should raise AnymailRecipientsRefused when *all* recipients are rejected or invalid"""
 
@@ -588,11 +591,13 @@ class MandrillBackendRecipientsRefusedTests(MandrillBackendMockAPITestCase):
         self.assertEqual(sent, 1)  # refused message is included in sent count
 
 
+@tag('mandrill')
 class MandrillBackendSessionSharingTestCase(SessionSharingTestCasesMixin, MandrillBackendMockAPITestCase):
     """Requests session sharing tests"""
     pass  # tests are defined in the mixin
 
 
+@tag('mandrill')
 @override_settings(EMAIL_BACKEND="anymail.backends.mandrill.EmailBackend")
 class MandrillBackendImproperlyConfiguredTests(SimpleTestCase, AnymailTestMixin):
     """Test backend without required settings"""