Poi.receivemail

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- Do more decoding. Each part can have its own encoding. We should
use those to make something readable. [maurits]

- Decode from quoted printable in a few cases. [maurits]

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- Moved to https://github.com/collective/poi.receivemail
[maurits]

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- Fixed wrong match for issue by subject. While searching for 123 in
the subject line we would match 123 at the beginning of the line as
well, which may give wrong matches.
[maurits]

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- Use the body charset to decode the body text and then encode it as
utf-8 before storing it.
[maurits]

- Avoid a unicode error when there is no body.
[maurits]

- Optionally add attachments from the e-mail. We look for binary
attachments. These are added as separate responses, as you can only
add one attachment per issue or response. In config.py we have
added a setting ADD_ATTACHMENTS for this (default True) to make it a
bit easier to switch this off in case of problems or when you do not
want it.
[maurits]

- Fixed handling multipart messages that have parts that themselves
are multipart.
[maurits]

- When there is an empty or missing Subject line, use '[no subject]'
as title. We do not try to match such a mail to a previous issue
but always create a new one.
[maurits]

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- Set more fields explicitly, as the createObjectByType call only sets
some of them.
[maurits]

----------------

- When a subject (keyword, category) is given, set that explicitly.
[maurits]

- In ``get_addresses`` allow getting addresses from headers other than
To and From as well.
[maurits]