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>
For dynamic cases like this, the resource could just manually set the `Links` and/or `Link-Templates` header. The [patch_response](https://github.com/cbmi/serrano/pull/234/files#diff-32fb8a620507cfd85ba7726f5c5741c3R141) method could just skip setting the header if it's already set.