handle absolute / relative paths and full URLs in links

fixes #1365

absolute paths start with a leading / and will
use only the provided host provided to the adapter

cf. { posts: [{ id: 1, links: { comments: "/posts/1/comments" }}] }

relative paths construct an URL from the record the
relationship is embedded in:

cf. { posts: [{ id: 1, links: { comments: "comments" }}] }

full URLs links will pass through the link transparently:

cf. { posts: [{ id: 1, links: { comments: "http://example.com/posts/1/comments" }}] }

note that all of the above examples are equivalent
to each other.
Merge pull request #1371 from tim-evans/master

Handle absolute / relative paths and full URLs in links