prepare for 0.18.0
add check-attachment hook
hitory, rel-notes: check-attachment
Display times in 24h format if configured so
readme: new mailing list: supmua@googlegroups.com
gem: new mailing list
readme: missing )
Merge #315: Display times in 24h format if configured so
prepare for 0.19.0
gpg: update keys for one more year and set up a script for regenerating them

Signed-off-by: Gaute Hope <eg@gaute.vetsj.com>