Merge pull request #312 from stefanbeller/master

Add requires_level and requires_not_level to NPC and Map.
Introduce a setting 'audio' in the config file to disable audio.

Currently this setting cannot yet be set in the userinterface config.
Having this option available, allows us to not call Mix_OpenAudio
if audio is completely disabled, so the audio subsystem is not initialized
at all.
Merge remote-tracking branch 'mainline/master'