Allow plugin "stateful" disconnection/exit.

[mpaperno]

• Adds 'exitOnClose' option to connect() method to control if client automatically calls process.exit(0) upon socket error/close (default is true).
• Adds 'disconnected' event with hadError <boolean> parameter passed on from socket's 'close' event (true if the socket had a transmission error).
• Adds 'socketError' event with err <Error> parameter passed on from socket's 'error' event.
• Adds disconnect() method which closes any current connection with socket.end().
• We don't need to exit() from multiple locations since socket's 'close' event is going to be fired regardless.

Just to note, if the only thing keeping the Node process active is the open socket to TP, then the plugin/program will exit anyway once the socket is closed/errors, regardless of the new 'exitOnClose' option. The only thing this option controls is literally if process.exit(0) is called automatically upon disconnection (or lack of successful connection in the first place).

[spdermn02]

Looks good