Adding an hdfs mode for the Scalding REPL

Added some code to the bash script wrapper and ScaldingShell. Users can now run the script as this:
```
USAGE: ./scripts/scald-repl.sh [-h] [-x local|hdfs]
   -h                   Show this help message and exit
   -x local|hdfs        Run REPL in local (default) or hdfs mode
```

This works in both local and HDFS modes. However, there is one annoying warning that I need to fix. The ILoop throws a warning - _bad option: '--hdfs'_

Comments @johnynek @ianoc? Right direction? Good, bad, ugly?