[4] Add linkdown command

* Linkdown command added to
  Client class. Tests still
  need to be written.

* distanceVector refactored to be
  Map<String, Double>, with the flag
  being removed from the functionality.
  Instead of a LINKUP/LINKDOWN flag, a
  weight of Double.POSITIVE_INFINITY is
  being used to symbolically represent a
  downed link (or a link of a link
  altogether) to more closely match the
  Dijkstra / Bellman-Ford standard.
[4] Test and debug linkdown()

* There was a bug in linkdown()
  where the key being used to
  lookup and populate the
  distanceVector was not properly
  formatted as "IPAddress:PortNum"

* Tests written for method, and
  are all passing.
Merge branch 'task-4-add_linkdown_command'