Subtle changes to run_tasks

 + run_tasks initializes itself the Database
 + BackgroundTasks implements a whereKeyIs which
   enables to do tasks on a single plugin by
   providing it's key.
   this method displays a message when
   triggered with an unexisting key
 + run_rask now has a command line api having
      -k  : to specify the key
      -t : to specify a task executed on the plugin found by key
      selector
   using BackgroundTasks->whereKeyIs() method
 + the run_tasks script fallback to the
   default set of tasks of no options is given
   by the command line.