Decorate httpClient response with duration property

[SimoneBergonzi]

The httpClient response is decorated with a new property duration indicating the time passed before the call was resolved.
relative issue

Checklist

• your branch will not cause merge conflict with master
• run npm test
• tests are included
• the documentation is updated or integrated accordingly with your changes
• the code will follow the lint rules and style of the project
• you are not committing extraneous files or sensible data

[coveralls]

Coverage decreased (-0.08%) to 97.55% when pulling 8f84150 on feature/http-call-duration into a044368 on master.

[fredmaggiowski]

LGTM