added /gm and gamemode

Signed-off-by: Hugo ThePooN Denizart <thepoon@cartooncraft.fr>
added usage message in godmodecommand

Signed-off-by: Hugo ThePooN Denizart <thepoon@cartooncraft.fr>
forgot to add the command to the plugin.yml .. ;_;

Signed-off-by: Hugo ThePooN Denizart <thepoon@cartooncraft.fr>