Add a simple singleton array cache implementation for use when memcached isn't an option

The patches add a simple singleton based array cache solution to act as a per process cache solution. Many processes in civicrm assumed caching when they were written and can make dozens if not hundreds of redundant database requests for common values such as Option Values and PsuedoConstants depending on the particular processing. By providing a simple caching solution these queries can be limited to once per request without introducing memcached as a project dependency.

Currently configured to activate with: CIVICRM_USE_ARRAYCACHE = 1 in the civicrm.settings.php file.