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_inbound.py

@@ -1,6 +1,6 @@
 from textwrap import dedent
 
-from django.test import override_settings
+from django.test import override_settings, tag
 from mock import ANY
 
 from anymail.inbound import AnymailInboundMessage
@@ -11,6 +11,7 @@ from .test_mandrill_webhooks import TEST_WEBHOOK_KEY, mandrill_args
 from .webhook_cases import WebhookTestCase
 
 
+@tag('mandrill')
 @override_settings(ANYMAIL_MANDRILL_WEBHOOK_KEY=TEST_WEBHOOK_KEY)
 class MandrillInboundTestCase(WebhookTestCase):
     def test_inbound_basics(self):