Split part tagging sent mail into separate class: SentMailsFilter

ArchiveSentMailsFilter inherits from SentMailsFilter.  SentMailsFilter
just tags message, ArchiveSentMailsFilter, in addition, archives
messages.
SentMailsFilter: Map To/Cc/Bcc addresses to specified tags 1:1

Transform To/Cc/Bcc e-mail addresses to tags according to the specified
rules.  It can be used for example to easily tag posts sent to mailing
lists which at this stage don't have List-Id field.
Merge branch 'sentmailsfilter'
SentMailsFilter: to_transforms support lists of tags
Manually merge the changeset.

Closes #52.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>