Fix improper header separator for X-OfflineIMAP header

For servers without UIDPLUS we are inserting additional header
just after transformation '\n' -> CRLF was done.  addmessageheaders()
was written to work with just '\n' as the separator, so X-OfflineIMAP
header wasn't preceeded by the CRLF, but just by '\n'.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Add version qualifier to differentiate releases and development ones

It is always good to see which version we're talking about, so I had
added explicit marker for -devel, -release, -rcX and other states of
the OfflineIMAP.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>