Add pagination information to API's json output

[webcracy]

Should be present in all outputs that are paginated.

Example:

{
  'data':  {
    'totalItems': 120,
    'items': [...]
  },
  'pagination':  {
    'current_page': 1,
    'per_page': 30,
    'next_page': 2,
    'previous_page': null,
    'total_pages': 4
  }
}