Various fixes mainly to do with validation / options

* Updated readme
* Grouped Bing, PinInterest, Yandex and Alexa validations as they use the same validation regex
* Made twitter card list a static class property for easy access from both validation and form and actually make it work (i.e. keys in form were numeric and would never validate)
* Simplified the twitter card validation
* Use filter rather than regex if regex is not needed (fbapps validation) and add this validation to the clean routine
* Remove superfluous `strip_tags()` as is already done in `sanitize_text_field()` (fb apps validation)
* Fix bug in fbadminapp validation
* Fixed `reset_ms_blog()`
* Fixed typo I made 'fbapps' not 'fb_apps'

Also:
DON'T nest what doesn't need nesting... changing `else if()`'s to `elseif()` hoping that will make your IDE behave....
Updated readme with enhancement added by Yoast
DON'T nest what doesn't need nesting... changing `else if()`'s to `elseif()` hoping that will make your IDE behave....

Also:
* Found a few more `plugins_url()` which didn't use WPSEO_FILE yet. Fixed.
* Some strict checking fixes === vs ==