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.
Need to look through the checks in `wp_new_comment()` and replicate them in this method, with good test coverage. Otherwise, total spam vector.