DRY common apipie param descriptions into param groups

Often there are the same params (or majority of them) on some actions, 
typically `create` and `update` on resource. New apipie-rails supports 
extracting this kind of params into param_group.

Also, params thare are required on creation, are not required, on update (only
can't be nil). `:action_aware => true` achieves this.