Django-sitemessage

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* Message.schedule() now respects message type priority attribute.
* `SITEMESSAGE_EMAIL_BACKEND_MESSAGES_PRIORITY` now defaults to None.

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* IMPORTANT: Package layout changed (messages, messengers locations changed).
* IMPORTANT: Changed default templates search path.
+ Implemented subscriptions handling.
+ Implemented Django email backend.
+ Implemented experimental `mark read` functionality for e-mails.
+ Implemented `List-Unsubscribe` e-mail header support for SMTP messenger.
+ Added Django 1.7+ migrations.
+ Added basic messages templates.
+ Added `INIT_BUILTIN_MESSAGE_TYPES` setting.
+ Added `EMAIL_BACKEND_MESSAGES_PRIORITY` setting.
+ Added `DEFAULT_SHORTCUT_EMAIL_MESSAGES_TYPE` setting.
+ Added `SITE_URL` setting.
* Fixed delivery errors logging.
* Dispatch model is now always passed into `compile()`.
* Exception handling scope broadened for Twitter.

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* Fixed `sitemessage_send_scheduled` command failure on Django 1.7.
* Now `time_dispatched` field value is timezone-aware.

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+ Django 1.7 ready.
+ Added Russian loco.
+ Implemented get_message_type_for_app() and override_message_type_for_app().

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+ Added Twitter messenger.
* Messengers are moved into a separate module.

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+ Added message priorities support.
+ Added support for sending test messages.
+ Now dispatches creation updates `dispatches_ready` message property.
+ Now message templates may use data from 'message_model' and 'dispatch_model' variables.
+ Now message is cached for dispatches with errors.