Add bob-walker in Scala

Had to be done, of course. It's not fully featured (no random, no drunk), and there are a number of improvements to the code to be made, but it's a start.

To test:

`scala bob-walker.scala -s`

I've just realised that it doesn't work without *any* command-line arguments, but it's too late at night now for me to fix that. It accepts command-line args either separately ('-d -s -x' etc) or together ('-dxs') and also the `-` is ignored, so:

`scala bob-walker.scala dsxp` for full glory. `^C` to stop, or it will stop eventually.