2.5 - Add Cache::remember()

I frequently use read-through caching strategies in my applications. This often ends up being repetitive code, that gets pushed into a helper function. I thought it might be useful to have this as a pre-baked workflow in Cache.

This new method allows you to provide callables that provide data for a cache key should the cache key not exist.

For example a model function could easily be used to cache results. Assuming you are using PHP 5.4 or later.

```php
<?php
class Articles extends AppModel {

  function all() {
    return Cache::remember('all_articles', function () {
      return $this->find('all');
    });
  }
}
```