Document rules for initialization/cleanup of ZTS globals.

Transfer stesie's comment from the c3512587acf2f6aeeb80d7cc753f08a784c4c86b
commit message so that future additions to v8js_globals maintain the
proper ZTS behavior.
Fix a bunch of memory leaks.

Found by compiling PHP with --enable-debug.
Fix memory leak: The get_properties handler owns the returned hashtable.
Fix double-pop of timer_ctx (and efree from wrong thread).

The timer_ctx was being popped & freed in terminate_execution, from the
timer thread (not the main thread), and then popped again in the main
thread when execution was aborted.  Do all pop/free work in main thread.
Travis config update: fix for issue #56.

Test against php 5.3, 5.4, and 5.5 -- which incidentally works around
the 'make test always succeeds' bug in php 5.3 and 5.4
(https://bugs.php.net/bug.php?id=60285).

('make test' may always succeed, but we still test against 5.3 and 5.4 to
ensure that we build correctly w/o errors on these php versions.)
Travis config update: compile against minimum supported v8 and most recent v8.