Apcu tabs

Adds support for [APCu PECL module](http://pecl.php.net/package/APCu).
1. Class ApcuHelper has most of [APCu'a apc.php](https://github.com/krakjoe/apcu) functions plus some helpers to draw images and list variables. The code has been altered to match PSR-2 standard but its functionality remains the same.
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.