Add support for Zend OPcache extension
- allow configuration of priority, because the opcache extension may have problems if it is loaded after xdebug extension
- set priority of opcache extension to 5, so it is loaded very early
- allow configuration of extension type normal/zend extension
- specify xdebug and opcache as zend extension
Merge branch 'squeeze-php54' of https://github.com/pl-github/dotdeb-php5-pecl into squeeze-php54