Added an (PHP) API for POST authentication codes.

The HTTP API is not added yet. Therefore I also didn't include the check anywhere.
The goal of the POST authentication code is to prevent attacks where a different
website submits a (hidden) POST form to change settings of a user.
Added the HTTP API to get the POST authentication code
Fixed BeeHub::startSession() so that it won't attempt to start a session when run from cli

This situation occurs during tests for example!
Added checks for the POST authentication code
Added the POST authentication code to all POST requests
add authentication code to post request without form
Merge branch 'master' of github.com:sara-nl/BeeHub