Make and test a Priority Queue.

This should be made so that it is easily adaptable to the shortest path
through a map assignment, but that does not require any extra work, it is
just a matter of good design.

Implement a struct or class that represents a place. All it needs to have
is a string for its name and an int for its priority (best distance known
so far). Implement a Priority Queue that stores pointers to those objects.

Make an interactive test, which takes commands from the user, and adds or
removes things from the queue accordingly.

The user may type one of three things:

  exit              stop the program

  add name prio     add a place with the given priority to the queue,
                    except that if a place with that name is already
                    in the queue, just change its priority (and adjust
                    its position), do not add a duplicate entry.

  remove            remove the place with the best priority from the
                    queue.

After each add or remove command, your program should print the entire
contents of the priority queue. The purpose is to allow you to test the
queue's functionality thoroughly before using it in a more complicated
project.

An example run (you do not have to duplicate this exactly):

> add a 50
queue contains
   a 50
> add cat 35
queue contains
   cat 35
   a 50
> add dog 70
queue contains
   cat 35
   a 50
   dog 70
> add b 40
queue contains
   cat 35
   b 40
   a 50
   dog 70
> remove
removed cat at 35, queue contains
   b 40
   a 50
   dog 70
> remove
removed b at 40, queue contains
   a 50
   dog 70
> add dog 45
queue contains
   dog 45
   a 50
> remove
removed dog at 45, queue contains
   a 50
> exit