Handle absolute / relative paths and full URLs in links

This fixes #1365

Absolute paths start with a leading `/` and will use only the provided host provided to the adapter.

```json
{ "posts": [{ "id": 1, "links": { "comments": "/posts/1/comments" }}] }
```

Relative paths construct an URL from the record the relationship is embedded in:

```json
{ "posts": [{ "id": 1, "links": { "comments": "comments" }}] }
```

Full URLs links will pass through the link transparently:

```json
{ "posts": [{ "id": 1, "links": { "comments": "http://example.com/posts/1/comments" }}] }
```

All of the above examples are equivalent to each other.