Added getPropertiesValue(server.properties, key), returns value of the key

This may need some work since it's valid for a key to exist with no
value. Currently this returns "--" if the key does not exist, and "" if
the key exists and there is no value.
Maybe I should check if my final edits are saved first
Added getWorldPropertiesValue(world, key)

Wrapper for getPropertiesValue which expects a file path to
server.properties so worldname can be used to get a server.properties
value
Support for per-world MOTD

Added the ability to use a worldname.motd file in
/home/minecraft/worlds/ when the script calls getMOTD() or tellMOTD()