Refactor replace vars: move away from singleton pattern

-> Changed a few properties and methods to static to make them available to all instances while only having to set them up once (efficiency). Applies to the registration of new replacement variables and the help text methods
-> Changed class calls to be in line with this change

Also: fixed issue where post title replacement was still playing up, this will probably fix the issues people using the Optima Express plugin were still having
Refactor replace vars: removed superfluous array cast as this is done on the receiving end anyways
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Replace-vars-no-more-singleton
Added filter `wpseo_replacements_filter_sep`