Add additional commands to the admin client.

Added the following methods to the GearmanAdminClient class
  1. get_jobs methods is equivalent to gearadmin's '--show-jobs'
  2. get_unique_jobs method is equivalent to gearadmin's '--show-unique-jobs'
  3. get_pid methos is equivalent to gearadmin's '--getpid'
  4. cancel_job method is equivalent to gearadmin's '--cancel-job'

The get_jobs, get_unique_jobs and cancel_job protocols has not been
released yet.  These protocols are in 1.2 branch commited in
milestone: https://launchpad.net/gearmand/+milestone/1.1.6