Rename callbacks to controller for AddingAPerson

* Just for consistency
Add first, naïve implementation for tagging autocompletion
Autocomplete support for taggings (WIP)
Improve handling of tagging autocompletion in case of non-existing entry
Add gem 'pry' to test and dev
Implement first_name, last_name, display_name and quick adding people

* 'name=' allows quick adding people.
  * Ex: user creation with name: 'Jan Dupond' results in an
  	user with first_name: 'Jean' and last_name: 'Dupond'

* virtual attribute 'name' now returns 'display_name' if set.
  Otherwise, it returns 'first_name'

* New attribue 'display_name' allows explicitely differentiating
  people with identical first_name, or using pseudonyms / familiar names