Add Mappers modules for making attributes conversion easier

When more processing is needed and blocks given to local attributes
become to long Mappers can be used. It's a module which is then
included in the remote object.
```
class Client < ActiveRecord::Base
  module SyncedMapper
    def first_name
      name.split(' ').first
    end

    def last_name
      name.split(' ').last
    end
  end

  synced mapper: SyncedMapper, data_key: nil,
    local_attributes: %w(first_name last_name)
end
```
`data_key: nil` is no longer needed ;-)