Use tox for running tests and building docs

* Set up tox for testing supported Django/Python combinations
* Also include tox env for checking and building docs
* Use tox-travis for Travis CI integration
* Add tests against Django master
* Document building docs and running tests with tox

tox.ini

@@ -0,0 +1,102 @@
+[tox]
+envlist =
+    # Test these environments first, to catch most errors early...
+    lint
+    django20-py36
+    django18-py27
+    docs
+    # ... then test all the other supported combinations:
+    django20-py{35,py3}
+    django111-py{27,34,35,36,py2}
+    django110-py{27,34,35,py2}
+    django19-py{27,34,35,py2}
+    django18-py{33,34,35,py2}
+    # ... then prereleases (if available):
+    #django21-py{35,36}
+    djangoMaster-py{36,37}
+
+[testenv]
+deps =
+    django18: django>=1.8,<1.9
+    django19: django>=1.9,<1.10
+    django110: django>=1.10,<1.11
+    django111: django>=1.11,<1.12
+    django20: django>=2.0,<2.1
+    django21: django>=2.1a1
+    djangoMaster: https://github.com/django/django/tarball/master
+    # testing dependencies (duplicates setup.py tests_require):
+    mock
+    sparkpost
+ignore_outcome =
+    django21: True
+    djangoMaster: True
+usedevelop = True
+args_are_paths = False
+commands =
+    python --version
+    # pip install .[mailgun,...,sparkpost]  ## usedevelop=True + manual deps is much faster on repeat runs
+    python runtests.py {posargs}
+passenv =
+    RUN_LIVE_TESTS
+    CONTINUOUS_INTEGRATION
+    MAILGUN_TEST_*
+    MAILJET_TEST_*
+    MANDRILL_TEST_*
+    POSTMARK_TEST_*
+    SENDINBLUE_TEST_*
+    SENDGRID_TEST_*
+    SPARKPOST_TEST_*
+
+[testenv:lint]
+basepython = python3
+skip_install = True
+passenv =
+    CONTINUOUS_INTEGRATION
+	# (but not any of the live test API keys)
+deps =
+    flake8
+commands =
+    python --version
+    flake8 --version
+    flake8
+
+[testenv:docs]
+basepython = python3
+skip_install = True
+passenv =
+    CONTINUOUS_INTEGRATION
+	# (but not any of the live test API keys)
+setenv =
+    DOCS_BUILD_DIR={envdir}/_html
+whitelist_externals = /bin/bash
+deps =
+    sphinx
+    sphinx-rtd-theme
+commands =
+    # Verify README.rst as used in setup.py long_description:
+    python setup.py check --restructuredtext --strict
+    # Build and verify docs:
+    sphinx-build -W -b dirhtml docs {env:DOCS_BUILD_DIR}
+    # Build README.rst into html:
+    /bin/bash -c 'python setup.py --long-description \
+                  | rst2html.py --config=docs/_readme/docutils.cfg \
+                  > {env:DOCS_BUILD_DIR}/readme.html'
+
+[travis]
+unignore_outcomes = True
+python =
+    3.6: py36, lint, docs
+
+[travis:env]
+DJANGO =
+    1.8: django18
+    1.9: django19
+    1.10: django110
+    1.11: django111
+    2.0: django20
+    2.1: django21
+    master: djangoMaster
+LINT_AND_DOCS =
+    true: lint, docs
+    docs: docs
+    lint: lint