[FEATURE] Add links for HATEOAS support

Initial patch to add the:

• Document link
• Relationship link
• Related link

Output with the patch applied (you can see tests as well):

{
  "data": {
    "type": "article",
    "id": "3",
    "attributes": {
      "nid": "3",
      "uuid": "c23ff989-8440-4ac2-a38c-0e232e0b120f",
      "vid": "3",
      "langcode": "en",
      "type": "article",
      "title": "Second article for testing",
      "status": "1",
      "created": "1463233887",
      "changed": "1463233940",
      "promote": "1",
      "sticky": "0",
      "revision_timestamp": "1463233922",
      "revision_log": null,
      "revision_translation_affected": "1",
      "default_langcode": "1",
      "path": null,
      "body": {
        "value": "<p>dummy dummy lorem<\/p>\r\n",
        "format": "basic_html",
        "summary": ""
      },
      "comment": {
        "status": "2",
        "cid": "0",
        "last_comment_timestamp": "1463233922",
        "last_comment_name": null,
        "last_comment_uid": "1",
        "comment_count": "0"
      }
    },
    "relationships": {
      "uid": {
        "data": {
          "type": "user",
          "id": "1"
        },
        "links": {
          "self": "http:\/\/d8dev.local\/api\/article\/3\/relationships\/uid?_format=api_json",
          "related": "http:\/\/d8dev.local\/api\/article\/3\/uid?_format=api_json"
        }
      },
      "revision_uid": {
        "data": {
          "type": "user",
          "id": "1"
        },
        "links": {
          "self": "http:\/\/d8dev.local\/api\/article\/3\/relationships\/revision_uid?_format=api_json",
          "related": "http:\/\/d8dev.local\/api\/article\/3\/revision_uid?_format=api_json"
        }
      },
      "field_category": {
        "data": {
          "type": "category",
          "id": "2"
        },
        "links": {
          "self": "http:\/\/d8dev.local\/api\/article\/3\/relationships\/field_category?_format=api_json",
          "related": "http:\/\/d8dev.local\/api\/article\/3\/field_category?_format=api_json"
        }
      },
      "field_image": {
        "data": null
      },
      "field_tags": {
        "data": [
          {
            "type": "tags",
            "id": "7"
          },
          {
            "type": "tags",
            "id": "5"
          }
        ],
        "links": {
          "self": "http:\/\/d8dev.local\/api\/article\/3\/relationships\/field_tags?_format=api_json",
          "related": "http:\/\/d8dev.local\/api\/article\/3\/field_tags?_format=api_json"
        }
      }
    },
    "links": {
      "self": "http:\/\/d8dev.local\/api\/article\/3?_format=api_json"
    }
  },
  "links": {
    "self": "http:\/\/d8dev.local\/api\/article\/3?_format=api_json&XDEBUG_SESSION_START=PHPSTORM"
  },
  "included": []
}