Adding a REPL

Adding a REPL to multitransport-jsonrpc so you can easily connect to a server and test the remote methods from the command line.

Don't merge yet, if this seems like a good idea, I'll need to write up a new section for the readme explaining how to use it (don't want to do that until the CLI app's command-line parameters are finalized).

cc @countrodrigo @squamos