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 Also setting these groups will allow permissions but then you need to make the controller authorize the actions. There are some commented out lines in there already that will helper with this and a brief review of the cancan page: https://github.com/ryanb/cancan/wiki/authorizing-controller-actions

And then there are those lines in the catalog_controller which setup gated access and make setting `read_groups` on objects actually work correctly: https://github.com/projecthydra/hydra-head/wiki/Lesson%3A-gated-discovery---filter-search-results-based-on-permissions