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/utils.py

@@ -8,32 +8,12 @@ import uuid
 import warnings
 from base64 import b64decode
 from contextlib import contextmanager
-from distutils.util import strtobool
 
 import six
 from django.test import Client
 from six.moves import StringIO
 
 
-def envbool(var, default=False):
-    """Returns value of environment variable var as a bool, or default if not set.
-
-    Converts `'true'` to `True`, and `'false'` to `False`.
-    See :func:`~distutils.util.strtobool` for full list of allowable values.
-    """
-    val = os.getenv(var, None)
-    if val is None:
-        return default
-    else:
-        return strtobool(val)
-
-
-# RUN_LIVE_TESTS: whether to run live API integration tests.
-# True by default, except in CONTINUOUS_INTEGRATION job.
-# (See comments and overrides in .travis.yml.)
-RUN_LIVE_TESTS = envbool('RUN_LIVE_TESTS', default=not envbool('CONTINUOUS_INTEGRATION'))
-
-
 def decode_att(att):
     """Returns the original data from base64-encoded attachment content"""
     return b64decode(att.encode('ascii'))