Add Test::Builder->store_events and use Test::More store_events => 1

Makes turning on events a little easier and it was easy to add.

Signed-off-by: Michael G. Schwern <schwern@pobox.com>
Consoldate the docs about event storage.

Also change the instructions for turning it on to use the new
Test::Builder->store_events method.

Signed-off-by: Michael G. Schwern <schwern@pobox.com>