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Streamline conversion process

Usage:
* Start server as usual, file with a list of worlds is created in
server root folder
* Shut down server
* Create command file in server root with:

```
#!/bin/bash
cd "$( dirname "$0" )"
java -Xmx1024M -jar plugins/mcMMO.jar
```

* Execute this file and it will start mcMMO’s converter - it will read
the file with a list of the worlds to convert and convert every one of
them