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_postmark_inbound.py

@@ -1,6 +1,7 @@
 import json
 from base64 import b64encode
 
+from django.test import tag
 from mock import ANY
 
 from anymail.exceptions import AnymailConfigurationError
@@ -12,6 +13,7 @@ from .utils import sample_image_content, sample_email_content
 from .webhook_cases import WebhookTestCase
 
 
+@tag('postmark')
 class PostmarkInboundTestCase(WebhookTestCase):
     def test_inbound_basics(self):
         raw_event = {