Replace embedded _links with Link(-Template) header

Fix #205. As noted in the conversation on #205 itself, this will take some updating by existing clients to read and process the Link and Link-Template sections from the header. The Link portion should be a very straightforward update as it behaves exactly as `_links` did but some more care will need to be taken when processing the Link-Template headers as they provide templates of URLs that will need to be adapted to individual objects based on id or other field replacements.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Just use `if header_links`