APCu tabs

Adds support for APCu PECL module.
1. Class ApcuHelper has most of apc.php functions plus some helpers to draw images and list variables
2. Tab Apcu Status: reports the current memory usage and provides the old apc.php graphs to showing memory segments. 
3. Tab Apcu Settings: lists the current runtime settings and adds an explanation of each one
4. Tab Apcu variables: lists the variables currently in apcu. This one will be enhanced in the near future. For now it's not possible to sort, print or delete the variable contents.