Collective.singing

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- Nothing changed yet.

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- support removing subscriptions that changed their email-address [fRiSi]

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- make channel description an optional field so users can leave it empty
if no description is needed
[fRiSi]

- Update README file to use github repo instead of svn.
[ivanteoh]

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- zc.queue should be less than 2.0.0a1 to not pull in ZODB4 and friends.
[ale-rt]

- fix TypeError on __cmp__ in AbstractPeriodicScheduler class.
[ivanteoh]

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- Made the last_trigger for the timed scheduler only record when there are newsletters sent.
[arterrey]

- Scheduler filter to send only to a channel option.
[arterrey]

- Cleanup flake8, changelog.
[WouterVH]

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- Add ``MANIFEST.in``.
[WouterVH]

- Added z3c.autoinclude-entrypoint to mark this as a Plone-plugin. This avoids
the need to explicitly load the zcml-slug.
[WouterVH]

- Fixed ImportErrors when zope.app.intid is not available but only
zope.intid.
[maurits]

- Added empty English translation, needed in corner cases with sites
that support multiple languages.
[maurits]

- Added Italian translations.
Refs https://bugs.launchpad.net/singing-dancing/+bug/466764
[andrea-cecchi85]

- Add IJob interface and let Job implements it.
[toutpt]

- Fix for plone 4.3
[kiorky].