allow deletion of last conversation post (#59)
use peek instead of last on conversation vector

peek is much more efficient for large vectors
remove conversations with only one status
modify 'last post in conversation' logic

1. Instead of checking to see whether a post is the last in the
conversation, check to see whether it has any replies. Otherwise, if I
respond to a post, and then someone else responds to the same post, I
can no longer delete my status.

2. Move all the logic about whether a user can delete a given post to
the same place. This a) simplifies the view logic, and b) allows us
to skip checking every status's `in-reply-to` field if it's unnecessary
(because if the user isn't the author, it can't be deleted anyway).
Merge pull request #60 from johnswanson/master

allow deletion of last conversation post (#59)