Added support to redefine users of a repository

Thanks, @scorphus :)
A route called `repository/grant` already exists and its purpose is different of `set`.
`grant` method just append new users to the list of users, ignoring the already existing users
`revoke` does the reverse, but also ignores the others too.
The `set` method, forces the users list to be the same as the users argument. Reseting all the user array. This method is important to garantee a correct updating permission flux in-one-action that normally will be implemented with `revoke` and `grant` methods, and can be dangerous if one of them does not works for some reason.