Feature/edit metadata authorization

I assume that pds_handle contain user group name(s) and thus we don't need to add roles. To reflect this I added customized method "groups" to user model. If the assumption is incorrect I will fix it and edit roles file.

@ekate I also prefer to use locales for messages: https://github.com/ryanb/cancan/wiki/exception-handling. Although this is not yet done for the other controller actions it's a good direction to go in.