Use heuristic to to auto-link relative paths and JSON urls.

  * If a key name hints that the value is probably a reference (i.e.,
    contains "href", "location", etc), and the value looks like a
    relative path, it's now linked.

  * If the current URL ends in ".json", and a relative link to another
    resource is created as above, append ".json" to the link URL.
    For example a link pointing to "/users/bob" will instead point to
    "/users/bob.json" if the current URL ends with ".json".  Makes
    traversing APIs that follow Rails-style conventions easier, but
    makes the relative autolinking less useful for much else.