Fix leak in v8 named property setter; clean up v8 named property getter.

zend_update_property does not own the value after the call; we need to
do the deref ourself.  Tweaks to the getter based on a better understanding
of the zend_read_property API.
Merge pull request #73 from cscott/a-few-more-leaks

Fix leak in v8 named property setter; clean up v8 named property getter.