Python: Remove dependency to requests

[tcalmant]

Requests is a good library, but its dependencies are problem when running on some interpreters or devices: six doesn't work "as is" with IronPython (it requires -X:Frames), requests requires the ssl module which isn't available on Arduino Yun, ...

It is necessary to replace requests by a homemade library:

• v1: make a Session class with a post method that mimics requests
• v2: implement a pool of connections to enhance performances