Simplify "scald-repl.sh"

Add `--tool` flag to `scald.rb` to enable the repl script to just be a simple wrapper to invoke `scald.rb` with the `ScaldingShell` tool.

This PR should also address #888 (add scalding subprojects to the classpath) because now any `scald.rb` flags will be passed through to `scald.rb` directly.

This does change the behavior of the repl slightly: `--hdfs` previously invoked the repl on a local hdfs instance, but now that flag is interpreted by `scald.rb` to be a remote hdfs which it will attempt to ssh to (instead, one should use `--hdfs-local`).

Questions to address yet:
- It seems that `--hdfs-local` was already "broken" by a change in Hadooop "provided" jars upstream. The script as it was worked around this by invoking `hadoop` directly, but a better solution, suggested by @ianoc, should be to fix this at the `scald.rb` level. Is it acceptable to break things temporarily with this and fix them separately in `scald.rb`?
- Should this logic to launch the repl just be folded into a flag to "scald.rb" instead of a whole separate script?