Lea user can delete own posts

Added delete to posts. Converted "allowed_to_modify?" and "is_owner?" methods to be take any object as an argument.

@garthgoldwater @amZotti 
So there won't be code outside of this model that calls `is_owner?`, right? It'll call `allowed_to_modify?` or `allowed_to_delete?`.

Therefore, I believe that `is_owner?` can and should be private.