Implement event objects as blessed scalar refs

The PV's data is the IB_EVENT structure

this avoids pointer/integer size mismatch warnings on some platforms
(https://buildd.debian.org/status/fetch.php?pkg=libdbd-firebird-perl&arch=armhf&ver=1.12-1&stamp=1376819741)
include event creation/destruction in 60-leaks.t