Python-postmark

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- Added message_id property to messages for post-send record-tracking (jdavisp3)

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- Added in .track_opens to support open tracking
See: http://developer.postmarkapp.com/developer-build.html#open-tracking
- Added POSTMARK_TRACK_OPENS setting to the Django backend (danxshap)
- Don't require `content_subtype` in Django backend to be set to `"html"` (danxshap)

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- Merged proper handling of content_subtype in the Django backend - thanks Josh Owen!

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- Merged in Python 3 support - thanks Jacob!
- Moving minimum Python version to 2.6. If you need 2.4 support, please continue to use v0.3.2!

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- Uses simplejson if installed for faster C _speedups
- PMBatchMail properly chunks large message sets
- Django backend uses PMBatchMail properly
- Use https by default
- Proper testing API key out of the box
- Cleaned up some PMBatchMail properties
- Added a few utility methods (add_message, remove_message) to PMBatchMail
- Added proper value checking to PMBatchMail pre-send
- A BIG THANKS to all new contributors!

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- Added batch-messaging support (PMBatchMail object) (deeGraYve)
See http://developer.postmarkapp.com/developer-build.html#batching-messages