Let users reply to comments

Previously: comments only replied to posts
Problem: reddit has really neat threaded conversations of comments replying to comments replying to comments, replying to (etc)
Solution: 
-make comments belong to a polymorphic "parent" which they are responding to (either posts or other comments)
-make the _comment partial recursive, so that a comment displays all its replies just below it in a nested list
maybe I just can't have a "reply to comment" box on every single comment. maybe you need to access that through going to the ```comment#show``` page and you can only respond to the post being shown there... until we add AJAX, at least. but this seems like something that should be feasible