add APCu Support

[dqualmann]

ZFDebug_Controller_Plugin_Debug_Plugin_Cache uses apc_cache_info() to read informations. This works fine for APC and APCu. But the information keys are different. So uses of Cache generates:

Notice: Undefined index: num_entries in vendor/jokkedk/ZFDebug/library/ZFDebug/Controller/Plugin/Debug/Plugin/Cache.php on line 103

Example for different keys:

APC	APCu
num_entries	nentries
num_hits	nhits
num_misses	nmisses
expunges	??

Tested with APCu 4.0.2 and 4.0.7.