Collective.singing

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- Used ``i18ndude`` to find all untranslated msgid's. Some line-numbers were
corrected, but nothing changed in ``collective.singing``. There were some
updates in collective.dancing, so all guy's please help updating
collective.dancing's po files. [saily]

- Add ``async`` module that allows to queue jobs for asynchronous
execution. Creating this was necessary because ``zc.async`` is not
available for all Zope versions that ``collective.dancing`` aims to
work with. :-( [nouri]

- Allow for more finer grained control of cue in
``IMessageAssemble.render_message``. This allows a preview function
to request the cue to be used but not to be set when rendering a
preview. [nouri]

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- Added optional checking for channel attr 'sendable' to
channel_lookup. Sending of a content item from the
sendnewsletter view in dancing uses this feature, to be able
to supress some channels. Also added vocabularies that contain
only sendable or only subscribeable channels.
[tmog]

- Relabelled buttons on the stats page to use the term 'clear' rather
than 'flush'. Also renamed related handlers and method. [russf]

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- Register the SubscriptionSearchableText adapter so that the fulltext
index is actually populated. [nouri]

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- Add 'headers' argument to ``mail.create_html_mail`` to allow the
injection of more e-mail headers. [nouri]

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- Add zope.sendmail as a dependency so that we get the latest and
greates release. [nouri]

- IMailDelivery expects ``toaddrs`` to be a list of addresses. [nouri]

- Use DumbWriters word wrapping to ensure that no text line
is longer than plain_text_maxcols characters. [saily]

- Using HTMLParser.anchorlist to append all links found in
a message at the bottom of the text/plain message. [saily]

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- Moved 'singing.scheduler.assemble_messages' and 'render_message'
into their own IMessageAssemble adapter (on channel). This allows
for easier hooking into these policy-riddled functions to make them
work differently for different types of channels. Also, added a
small flag that allows to ignore the subscriber's 'cue' completely.
[nouri]