[B] Fixes the Duration-Value in /effect command, Fixes BUKKIT-3967

The Problem is that the Duration time that is outputted is in ticks and not in seconds.

I fixed that by adding a variable (duration_sec) that contains the duration in seconds. This Variable is initalized by

duration_sec = duration / 20;

and replaces the duration variable in the broadcastCommandMessage-Command.

BUKKIT-3967 - https://bukkit.atlassian.net/browse/BUKKIT-3967