Refactor pager helper
XML DOM helper accepts more than one filter callback

Remove also compatibility issue with PHP 5.2
Refactor client data to support afs:trunc
Remove curl option fail on error to get better error message
Refactor response helper:

- Direct access to spellcheck results instead of use of intermediary
  manager.
- Delay until format method is called when transforming result in array
  format (except for facet values).
Update documentation