Implement basic task management

[ForNeVeR]

Implement basic task management as described in the tasks RFC (#6).

• add a simple data storage for Praefectus (e.g. a JSON file)
• add simple commands to
  • show list of tasks
  • create a task
  • create attribute
  • delete an attribute
  • set attribute value of a task
  • perform diagnostics, as defined by RFC
    • determine invalid task references (either non-existing ones or Deleted)
    • determine invalid attribute values
• verify that the implementation corresponds to the specification

Depends on:

• Simplify the task management system #39