implemented purge_cache

* new public static method `purge_cache()` in `LaterPay_Helper_Cache`
* support for: w3totalcache, wp super cache, quick cache, cachify, 
* added purge cache to several `update_option`'s
* added purge cache to category-database for insert/update/delete
* added custom action `laterpay_purge_cache` to enable the support for other cache plugins:

```php
add_action( 'laterpay_purge_cache', 'demo_custom_purge_cache' );
function demo_custom_purge_cache(){
  // my custom purge cache ...
}
```

references to #401