Implement better requirements checking

* Bring in Aihrus Framework
* Create constants out of requirements
* Remove unused methods
* Separate checking for active plugins and PHP from plugin versions
* Simplify class constants
BUGFIX #3
Issues update

* BUGFIX #1
* BUGFIX #2
* FEATURE #6 Enable donate_notice
* WONTFIX #3 Not needed
* WONTFIX #6 Needed
WONTFIX #5 Needed
Remove lib/aihrus again
BUGFIX Donate notice shows after saving settings

* BUGFIX Donate notice shows after saving settings
* Update readme regarding buy Premium for date and numeric fields
Implement WordPress version checking
Squashed 'lib/aihrus/' content from commit 639684c

git-subtree-dir: lib/aihrus
git-subtree-split: 639684c58140e0b2711958c4a973e230d9ba4e0c
Merge commit 'e0ca0fab4f83bdef0478ef620e9170057c20ffe2' as 'lib/aihrus'