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_mailjet_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 anymail.exceptions import (AnymailAPIError, AnymailSerializationError,
                                 AnymailUnsupportedFeature,
@@ -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('mailjet')
 @override_settings(EMAIL_BACKEND='anymail.backends.mailjet.EmailBackend',
                    ANYMAIL={
                        'MAILJET_API_KEY': '',
@@ -64,6 +64,7 @@ class MailjetBackendMockAPITestCase(RequestsBackendMockAPITestCase):
         ])
 
 
+@tag('mailjet')
 class MailjetBackendStandardEmailTests(MailjetBackendMockAPITestCase):
     """Test backend support for Django standard email features"""
 
@@ -354,6 +355,7 @@ class MailjetBackendStandardEmailTests(MailjetBackendMockAPITestCase):
             self.message.send()
 
 
+@tag('mailjet')
 class MailjetBackendAnymailFeatureTests(MailjetBackendMockAPITestCase):
     """Test backend support for Anymail added features"""
 
@@ -623,11 +625,13 @@ class MailjetBackendAnymailFeatureTests(MailjetBackendMockAPITestCase):
             self.message.send()
 
 
+@tag('mailjet')
 class MailjetBackendSessionSharingTestCase(SessionSharingTestCasesMixin, MailjetBackendMockAPITestCase):
     """Requests session sharing tests"""
     pass  # tests are defined in the mixin
 
 
+@tag('mailjet')
 @override_settings(EMAIL_BACKEND="anymail.backends.mailjet.EmailBackend")
 class MailjetBackendImproperlyConfiguredTests(SimpleTestCase, AnymailTestMixin):
     """Test ESP backend without required settings in place"""