Comments Endpoints

Still a Work In Progress, but opening to show progress and encourage any feedback as this progresses.

I took the (controversial) decision to use /comments as the canonical place for comments. I was then thinking `/posts/1/comments` would 301 to `/comments?post_id=1` or somehting along those lines. I think it's nicer to keep the comments at their own top level so they are independant of posts apart from the embedding we are doing elsewhere already.
Should this be `post_id` ?