Add/update/delete permission targets.

:construction: 

WIP for supporting permission targets.

- [ ] tests

cc @opscode/release-engineers 
We could probably just expose `PermissionTarget#users` and `PermissionTarget#groups` methods to make setting and getting the values easy (along with a `principals` attribute):

```ruby
def users
  principals['users']
end

def users=(users)
  principals['users'] = users
end
```