Add support for scalas & screpl (as copies of / symlinks to sbt)

Hey Paul,

I've just started to play with [scalas & screpl](http://www.scala-sbt.org/release/docs/Detailed-Topics/Scripts) and I thought it could make sense to support them in your script :-)

```
ln -s sbt scalas
ln -s sbt screpl
```

Cheers