Provide an Iterator to iterate over all the members of groups

This is about adding a new API in RightsManager to be able to iterate with an Iterator. Right now the API that exists is resolveUsers() and it returns a Collection. Thus if you have 1000 members of a group you'll have to wait till the 1000 docs are loaded before returning. Some code may want to break this into small step and an iterator will allow this (for example if you wish to have a progress bar or in my case to be able to send emails to all users of a group and be able to see the status for each mail before all have been sent).

See also http://markmail.org/message/7e4e6brinqbq3lzc