Support for different cache systems and choice thereof during installation [memcache,apc]

Cache class rewritten as a wrapper for cache drivers in cache_driver to allow easy addition and modification of cache systems without changes to the core, in the same way one can add database drivers.
Additionally installation step_3 was modified to allow the user to choose from available cache systems and enter configuration (APC, memcache, File cache, based on what is available in PHP). It would undoubtedly be better to hide memcache config when memcache is not selected, but that would require pulling in jquery which seems unnecessary. Now memcache configuration options[host, port] are shown whenever Memcache is available in php, which may be somewhat confusing.
Minor changes were made to the original cache, namely adding KEY_PREFIX to cache file names, to remain consistent with other cache systems and to allow sharing of the same cache folder between different applications, renaming to Cachedefault and moving to cache_driver/default.php