Postman collections and tutorial to test Creme_API

[MorganeAD]

This PR depends on the Creme_API PR of @hsmett, here the link #380

I open another PR to make it easier to review.

In this PR you will find:

• a Postman collection of all endpoints of Creme_API,
• a Postman environment file to use it in localhost,
• a README file to explain how to use Postman with our collection. You can see the tutorial better using this link https://github.com/HybirdCorp/creme_crm/tree/MorganeAD-creme_api_postman/creme/creme_api/tests/postman#overview

[MorganeAD]

thanks @genglert for the english corrections :)