Add SparkPost support (#20)

Implement SparkPost backend and tracking webhooks.

Closes #11.

.travis.yml

@@ -26,6 +26,8 @@ cache:
 install:
   - pip install --upgrade setuptools pip
   - pip install $DJANGO
-  - pip install .
+  # For now, install all ESPs and test at once
+  # (in future, might want to matrix ESPs to test cross-dependencies)
+  - pip install .[mailgun,mandrill,postmark,sendgrid,sparkpost]
   - pip list
 script: python setup.py test