[5] Add linkup command and tests

* The linkup() command has been
  added to the Client class to
  allow a link that has been
  previously torn down (so it
  has been given weight infinity)
  to be 'reset' with a weight given
  as a parameter.

* Unit tests written

* Several helper methods have
  been added to the Client class as
  well to aid in testing, and allow
  for cleaner tests.
Merge branch 'task-5-add_linkup_command'