Dynamically set attr_accessible for all defined SeoMeta attributes.

Recently I had a need to add back `meta_keywords` to SeoMeta. I struggled with properly decorating `Refinery::Page::Translation` to set `attr_accessible :meta_keywords` so I came up with a simple solution and IMO it's even better to not hardcode SeoMeta attributes but instead access them dynamically.

@parndt maybe I'm overlooking something so I need your opinion here :)