add fileEncoding config option to OS X runner

When running Jenkins under OS X in Japanese (or also in other non-eurpoean languages I assume), non-ASCII characters in Jenkins console output becomes question ("?") sign.
By applying this patch, users can now easily configure `file.encoding` JVM parameter for Jenkins by running:

```
sudo defaults write /Library/Preferences/org.jenkins-ci fileEncoding UTF-8
```

... instead of doing messy tweaks like [using JAVA_TOOL_OPTIONS](http://blog.lidalia.org.uk/2011/04/setting-default-java-file-encoding-to.html) or [modifying system global launchd.conf file](http://stackoverflow.com/questions/135688/setting-environment-variables-in-os-x/588442#588442), to make console messages correctly displayed.