Allow the NetLogo shell to be invoked from any directory

Currently, `bin/shell.sh` must be invoked from the NetLogo root directory. This fixes that. Thus, you can add the shell to your `PATH`, `cd` to a directory with a model, and run `netlogo-shell my-model.nlogo` (or whatever) to open a repl for the model.

I'm using this to quickly test extension commands.