Lots of improvements + more test cases

* BREAKING CHANGE: Default value for `updated_accessor` was `:updator`, is now `:updater`, since "updator" is not an English word
* DEPRECATION: #configure` is now an alias `#config` and is deprecated
* Replace `Mongoid::Userstamp.configuration` and `Mongoid::Userstamp.configure` with `#config`, which is both a getter and a setter
* Replace usages of `id` with `_id`, since there are many gems which override the behavior of `id`, for example mongoid_slug
* Do not overwrite creator if it has already been set manually
* Define setter methods for updated_accessor/created_accessor which assign the user id. Accepts both Mongoid::Document and BSON::ObjectID.
* Allow pass-in of Mongoid field opts for updated_column/created_column. Useful to set field aliases, default value, etc.
* Expose current_user as Mongoid::Userstamp.current_user
* Field type is now ::Moped::BSON::ObjectID instead of Object
* Catch error if creator/updater has been deleted and return nil
* Query for creator/updater user using `unscoped`
* Improve code readability
* Improve test coverage, including adding tests for config