Add contrib script nmcsh - simple namecoind readline wrapper

This provides history (Ctrl-p, Ctrl-n) command completion (TAB on an
incomplete command), argument help (TAB after a finished command),
and the other usual readline keys. See
http://tiswww.case.edu/php/chet/readline/rluserman.html for
details.

Before running this, make sure namecoind is in your $PATH and the
dameon is running.