Run REPL from SBT

Closes #1008.

This allows the actual custom Scalding REPL to be run directly, without building the full assembly: `./sbt scalding-repl/run`.

This required some SBT magic to add provided dependencies back in for the repl's run command, and a modification to the interpretter settings to inherit the right settings from SBT.

I also made `--local` default, but this change could easily be reversed if anyone felt strongly about being explicit, but I felt since it still won't do the magic that `scald.rb` does to make it remote, this use case is pretty heavily weighted to running `local` mode.