#5 patch for abook_group

[Tomas Kuliavas]

Adding patch from abook_group v.0.50 mailbox file.

Original author: Willem Jaap Zwart <W.J.Zwart at
NescioLudens.nl>
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Besides a small change in the abook_group_interface a
number of changes were needed in compose and the
Rfc822Header class.

The interface just adds the group name to one of the
fields (cc, to, bcc) and the compose and rfc822 files
take care of resolving this name to the group members
on delivery.

At least one thing I need to implement additionally:
group names should not be allowed to contain whitespace
anymore.

[Tomas Kuliavas]