Implement server for the REST API

[memowe]

Closes #10.

• Simple "Hello world" example with API and server separation and tests

API implementation:

• GET /person
  • List persons
• GET /person/:id
  • Person including assigned tasks (including assignees)
• POST /person
  • Person creation, returns the person data including ID
  • Req: {"name": "Bar Baz", "email": "bar@baz.org"}
  • Res: {"pid": 42}
• GET /task
  • List of top-level tasks (including assignees)
• GET /task/tree
  • Full task forest
• GET /task/:tid
  • Details of task including subforest
• POST /task
  • Top-level task creation, returns the task data including TID
• POST /task/:tid
  • Subtask creation, returns the task data including TID