* Create generic TestBackend that simply collects
send parameters
* Change BackendSettingsTests to TestBackend,
and add some missing cases
* Add UnsupportedFeatureTests
* Replace repetitive per-backend SEND_DEFAULTS
test cases with single (and more comprehensive)
SendDefaultsTests
* message.template_id to use ESP stored templates
* message.merge_data and merge_global_data
to supply per-recipient/global merge variables
(with or without an ESP stored template)
* When using per-recipient merge_data, tell ESP to use
batch send: individual message per "to" address.
(Mailgun does this automatically; SendGrid requires
using a different "to" field; Mandrill requires
`preserve_recipients=False`; Postmark doesn't
support *this type* of batch sending with merge data.)
* Allow message.from_email=None (must be set after
init) and message.subject=None to suppress those
fields in API calls (for ESPs that allow "From" and
"Subject" in their template definitions).
Mailgun:
* Emulate merge_global_data by copying to
recipient-variables for each recipient.
SendGrid:
* Add delimiters to merge field names via
esp_extra['merge_field_format'] or
ANYMAIL_SENDGRID_MERGE_FIELD_FORMAT setting.
Mandrill:
* Remove Djrill versions of these features;
update migration notes.
Closes#5.
Treat Mailgun metadata like all other ESPs: simple
key-value dict, where values are strings. If you want
to store JSON in metadata, you should serialize and
deserialize it yourself.
