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 Could you put a comment as to what is going on here?

I think that setting the edit group to 'public' on newly created items means that any users, regardless of being logged in or not, can edit this item, per https://github.com/projecthydra/hydra-head/wiki/Lesson:-Quick-Overview-of-Access-Controls-in-Hydra#two-special-types-of-groups-registered-and-public