Retrieve TagHierarchy with a tree sructure

[Guillem96]

Create new api request:

• Method: GET
• URI: tagHierarchies/{id}/tags
• Response:

  { "name" : "tagHierarchy1",
  "roots" : [
  { "name" : "tag1",
     "children" : [
          {"name" : "tag2",
            "children" : [
                ...
             ]
          },
          ...
      ]
  }
  , ...
  ]
  }