patch to add \watch to psql

This patch adds \watch to psql. It is much like the unix equivalent,
defaulting to every 2 seconds, and allowing you optionally specify a number
of seconds.

I will add this to the commit fest app.

Thanks,
Will Leinweber

Example:

psql (9.3devel, server 9.1.4)
Type "help" for help.

will=# \watch select now();
Watch every 2s Fri Oct 19 17:09:23 2012

              now
-------------------------------
 2012-10-19 17:09:23.743176-07
(1 row)

Watch every 2s Fri Oct 19 17:09:25 2012

              now
-------------------------------
 2012-10-19 17:09:25.745125-07
(1 row)

Watch every 2s Fri Oct 19 17:09:27 2012

              now
-------------------------------
 2012-10-19 17:09:27.746732-07
(1 row)

^Cwill=# \watch 5 select now();
Watch every 5s Fri Oct 19 17:09:33 2012

              now
-------------------------------
 2012-10-19 17:09:33.563695-07
(1 row)

Watch every 5s Fri Oct 19 17:09:38 2012

              now
-------------------------------
 2012-10-19 17:09:38.564802-07
(1 row)

^Cwill=# \watch select pg_sleep(20);
^CCancel request sent
ERROR:  canceling statement due to user request
will=#