Add requirements helper

* Abstract notice helper methods as functions
* Add requirements helper
* Switch from require_once to require file inclusion
* Use `aihr_` as function prefix
Remove unused methods
FEATURE #1 PHP Version checking

* Add aihr_notice_license
* Add name filed to aihr_check_php
* Encase pronouns in double-quotes
* FEATURE #1 PHP Version checking
* static::ITEM_NAME to static::NAME
FEATURE #2 Add WordPress version check