Patch fixes:

1) Adds default handler mechanism.  The user should register default
handlers under the "default" pseudo-performative:

	kqml register_script default { puts "foo" }

This default handler will be called when a message arrives for which
there is no other handler.

Bugs fixed:

behavior of get_ith_field fixed.

handlers that don't return a message, when the message has a
reply-with field, no longer cause crashes.