Compatibility with Python 2.7 versions older than 2.7.7
* Use Django's constant_time_compare method
* Include sparkpost in test requirements
* Don't use non-public `EnvironmentVarGuard` in tests
Fixes#41
Drop `from __future__ import unicode_literals`;
it was there for Python 3.2 compatibility (which
Anymail doesn't support). Ensures tests use normal
strs in Python 2.x.
When using a stored template, SparkPost disallows
subject, text, and html. Django's EmailMessage default
empty strings are enough to provoke "Both content
object and template_id are specified" from SparkPost,
so remove them (if empty) when using stored templates.
Update docs and tests; add integration test for template_id.
Fixes#24
python-sparkpost generates a transmissions.send
payload which is now considered invalid by the API,
if you try to use both `cc` (or `bcc`) and the
`recipients` dict structure required for merge_data.
[Anymail had been generating that recipients dict
structure in all cases, for simplicity. Sometime
between 2016-06-07 and 2016-06-22, SparkPost
began rejecting that if it appeared in the `header_to`
constructed by python-sparkpost.]
