fixes #71 and improves the way nether decay is handled.
Merge branch 'master' of https://github.com/trichner/Metropolis

# By trichner
# Via trichner
* 'master' of https://github.com/trichner/Metropolis:
  fixed abstract class having implemented an abstract method
  changed default NoLagg fixing to false
  changed api exception, throws now an Exception if the plugin is missing and the option is on