Add 'Description' and 'MergedTickets' fields

[ruicleite96]

This PR adds two fields to the ticket search endpoint:

• Description: Content of the ticket first transaction;
• MergedTickets: List of the tickets merged into the current one.

Example:
http://localhost/REST/2.0/tickets?query=Queue='general'&fields=Description,MergedTickets

{
  "pages": 1,
  "page": 1,
  "per_page": 20,
  "count": 1,
  "items": [
    {
      "id": "652",
      "Description": "<p>Ticket description text...</p>",
      "type": "ticket",
      "MergedTickets": [
        "650",
        "541"
      ],
      "_url": "http://localhos/REST/2.0/ticket/652"
    }
  ]
}