[Enhancement] Using Links vocabulary to describe the TD location

It is possible to describe in which resource of the servient the TD is served by using the Links vocabulary, i.e. in the Thing level, the TD can have:
```json
{
"links": [
{
"rel": "self",
"href": "http://localhost:8080/counter/",
"type": "application/td+json"
},
{
"rel": "self",
"href": "coap://localhost:5683/counter/",
"type": "application/td+json"
},
{
"rel": "self",
"href": "mqtt://test.mosquitto.org/counter/",
"type": "application/td+json"
}
]
}
```
I think it can be a good addition since there is the implicit assumption that a node-wot TD is stored at `IP/ThingName` but this is not communicated anywhere. When the TD is communicated to others (in Plugfests for example), it would be good that everyone knows where to find the TD.