Add extension YAC (Yet Another Cache)

I'd like to see the extension YAC (Yet Another Cache) added to homebrew-php.

Since Optcode Caching is done in the PHP core, Xinchen Hui (also known as laruence) developed an extension just containing the user-cache.

> It can be used as a replacement to APCu. You just need to rename the apc-calls to yac-calls.

Some benchmarks and further information can be found here:
- http://www.laruence.com/2013/03/18/2846.html
- https://github.com/laruence/yac