Zend Engine 3

I think it's worth considering bumping the version number for ng. Changes may be yet to come, sure, but phpng was such a big overhaul I think it's worth calling "Zend Engine 3".

I haven't updated extensions which check for the constant, but here's a list from grep:

    oa-res-27-90:php-src ajf$ grep -r ZEND_ENGINE_2 .
    Binary file ./.git/objects/pack/pack-779abadf31d95ebd50611dbd5e7f4670f60dd109.pack matches
    ./ext/mysql/php_mysql.c:#ifdef ZEND_ENGINE_2
    ./ext/mysql/php_mysql.c:#ifdef ZEND_ENGINE_2
    ./ext/mysql/php_mysql.c:#ifdef ZEND_ENGINE_2
    ./ext/oci8/oci8.c:#ifdef ZEND_ENGINE_2
    ./ext/oci8/oci8.c:#else /* ZEND_ENGINE_2 */
    ./ext/oci8/oci8.c:#endif /* ZEND_ENGINE_2 */
    ./ext/pdo_firebird/firebird_driver.c:#ifdef ZEND_ENGINE_2
    ./ext/xml/xml.c:/* #ifdef ZEND_ENGINE_2
    ./ext/xmlwriter/php_xmlwriter.h:#ifndef ZEND_ENGINE_2
    ./ext/zip/zip_stream.c:#ifdef ZEND_ENGINE_2
    ./ext/zip/zip_stream.c:#endif /* ZEND_ENGINE_2 */
    ./main/internal_functions_win32.c:#ifndef ZEND_ENGINE_2

That'd make sense, would avoid updating existing extensions. Maybe we should add a comment for clarity in that case, though? Something like this:

    /* ZEND_ENGINE_2 is also defined for backwards-compatibility
     * Therefore, do not assume that because ZEND_ENGINE_2 is defined, it is ZE2
     * This only guarantees it is ZE2 or later
     */