Undefined index errors for /wp-json.php/posts/POST_ID/comments

Fixes the first issue described on GSoC #382: http://gsoc.trac.wordpress.org/ticket/382

There are undefined index errors thrown in the prepare_comment() function due to $post never being defined.

There is also an instance of `$post->comment_date_gm`t that is supposed to be `$comment->comment_date_gmt`.

Adding `$post = (array) get_post( $fields['post'] );` at line 966 of class=wp-json-posts.php fixes the undefined index.

And changing line 1033 of class-wp-json-posts.php from` $fields['date_gmt'] = date( 'c', strtotime( $post->comment_date_gmt ) );` to `$fields['date_gmt'] = date( 'c', strtotime( $comment->comment_date_gmt ) );` fixes the second issue.