Replace all references to CCK with Fields

CCK doesn't exist in D7 except as an upgrade path. It's already going to be confusing, and having contrib reinforce the difference is important.

Looking through the code, you use all the Field API functions properly. And I really like the usage of metatags_quick_field_validate() ;)

All you really need to do is change the description on the project page, and change the Category of the module.