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>
Resource instances are treated as the view passed into the `urlpatterns` and so they are shared between many requests. The issue with it being an instance is that can hold state if we are not careful. This line is not thread-safe since this could be set by another request before [this line](https://github.com/cbmi/serrano/pull/234/files#diff-cfd99515ea53816f8d424f65ccc974cbR213) is called. We *could* add a lock, but I would rather think of a different approach that would prevent needing to do a runtime set like this.