Add tree view for children

[allan-simon]

To be merged after pull request #7 it permits to have output like this

Title: [API] Definition & Deployment
Status: New
Author: XXXX
description:
List of tasks to be able to get the API blablabla 

├── [6694] blabla
│  ├── [6695]  user_login
│  │  ├── [6696] Define user_login
│  │  ├── [6697] Deploy user_login
│  │  ├── [6828] Implementation of user_login
│  │  └── [6829] Implementation of user_login
│  ├── [6698] forgot_password
......

[allan-simon]

I've corrected a small mistake as in python3 relative import only support the from . syntax, should be good to merge now