Added rereddit.unread function.
See example usage below.

    rereddit.login('username', 'password').end(function(err, user) {
      rereddit.unread().as(user)
        .end(function(err, unread) {
          console.log(unread);
        });
    });
Merge pull request #1 from zachfogg/master

Added a function to request a user's unread messages.