Introduce CountryCode validator (rework of #4142)

This is an update to #4142; I rebased the original branch and updated to include feedback on the initial prototype: 

 - Bump copyright year + CS fixes
 - Remove unused resource bundle files
 - Removed hard-coded country names
 - Use `static` instead of `self` in `Zend\I18n\CountryDb`

There is one outstanding issue that I'm unsure of:  country codes are not locale-specific, so should this code (or at least the Validator class) be moved out of the I18n namespace?  If so, where should the resource class (`Zend\I18n\CountryDb`) be placed?

Thanks to @mwillbanks for the bulk of the work on this and also to those who left their feedback in the original PR (#4142)