Add openid AX schema definition for external use

OpenID AX support is maybe not something for core, but this patch would be really helpful for the contrib world of openid modules.
http://www.axschema.org/types/
Basically this patch helps to eliminate code duplicates of the modules in the openid stack: http://groups.drupal.org/node/24033
Drupal contrib supports to act as a relying party or a provider. That's why this AX schema would be duplicated if it's not in the core. Also, if in the future, limited AX support will be included in the core, this schema is needed for that.

Fortunately D7 file registry makes possible that this patch is only a single .inc file and an additional line in .info.
All you need to do is include this file and use the definition array.