Implement hypertext cache pattern.

* withRepresentation now checks HYPERTEXT_CACHE_PATTERN flag
  and adds embedded resource's 'self' link to links with same rel.
* See https://tools.ietf.org/html/draft-kelly-json-hal-06#section-8.3
Merge pull request #19 from petejohanson/hypertext_cache_pattern

Implement hypertext cache pattern.