Don't need to print ex in default_actions.
Don't read new input unless we're done with the last repl message.

Rather than looping back to Readline.read after every "done" message,
we inspect the message and only do so if it's the result of a
top-level repl message. Without this we get input and output out of
sync when sending non-repl messages such as stdin or interrupt.

Fixes #11.
Exit when reading Eof from nrepl socket.
Don't suppress output in exit_actions.