Security

Yan,

I tweaked the security policy file used by [scripster](https://github.com/razie/scripster) to make all examples on scala-tour.com pass except the Remote Actors example.

Please evaluate. If I get a chance sometime next week, I'll look into the changes necessary to make the Remote Actors example work properly

This policy file should be a bit more robust than a whitelist as mentioned in yankay/scala-tour#6, because it allows you to whitelist not only imported packages, but file access, network access, and other things.

To edit this `policy` file, just execute `policytool` and you'll have a gui for editing the file.

Thanks,

Jim