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

runtests.py

@@ -4,7 +4,9 @@
 #   or
 # runtests.py [tests.test_x tests.test_y.SomeTestCase ...]
 
+from __future__ import print_function
 import sys
+from distutils.util import strtobool
 
 import django
 import os
@@ -17,6 +19,18 @@ def setup_and_run_tests(test_labels=None):
     """Discover and run project tests. Returns number of failures."""
     test_labels = test_labels or ['tests']
 
+    tags = envlist('ANYMAIL_ONLY_TEST')
+    exclude_tags = envlist('ANYMAIL_SKIP_TESTS')
+
+    # In automated testing, don't run live tests unless specifically requested
+    if envbool('CONTINUOUS_INTEGRATION') and not envbool('RUN_LIVE_TESTS'):
+        exclude_tags.append('live')
+
+    if tags:
+        print("Only running tests tagged: %r" % tags)
+    if exclude_tags:
+        print("Excluding tests tagged: %r" % exclude_tags)
+
     warnings.simplefilter('default')  # show DeprecationWarning and other default-ignored warnings
 
     # noinspection PyStringFormat
@@ -25,7 +39,7 @@ def setup_and_run_tests(test_labels=None):
     django.setup()
 
     TestRunner = get_runner(settings)
-    test_runner = TestRunner(verbosity=1)
+    test_runner = TestRunner(verbosity=1, tags=tags, exclude_tags=exclude_tags)
     return test_runner.run_tests(test_labels)
 
 
@@ -36,5 +50,30 @@ def runtests(test_labels=None):
     sys.exit(bool(failures))
 
 
+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)
+
+
+def envlist(var):
+    """Returns value of environment variable var split in a comma-separated list.
+
+    Returns an empty list if variable is empty or not set.
+    """
+    val = os.getenv(var, "").split(',')
+    if val == ['']:
+        # "Splitting an empty string with a specified separator returns ['']"
+        val = []
+    return val
+
+
 if __name__ == '__main__':
     runtests(test_labels=sys.argv[1:])