Q:

How to synchronize VSTS and Github respositories when commits are made

I have a repository in visual studio team services that I would like to keep synchronized with a github repository.  
This allows me to do my main development in VSTS and when merging into master it will be synced to github and also allow others to contribute on github and when there Pull Requests are merged into master its synced to VSTS. 

A:

First create a new build on VSTS that is using the repository that should be synced from VSTS:

add two CMD tasks that will run some git commands. 

where the last of the two needs a personal access token from Github. 
In the images both CMD tasks uses the GIT tool and the following two commands:
pull https://github.com/s-innovations/MessageProcessor.ServiceFabric.git master

and 
push https://$(githubpersonaltoken)@github.com/s-innovations/MessageProcessor.ServiceFabric.git head:master

Enable the CI option to trigger the build to run whenever something is commited to master. 
Now the same can be done the other way, where a new build is made the same way but with the urls changed to target visual studio online repository. 

Do note that when using personal tokens on vsts the authentication part of the url needs to be https://:token@ and on github its just https://token@.
push https://$(vstspersonaltoken)@sinnovations.visualstudio.com/DefaultCollection/S-Innovations%20MessageProcessor/_git/messageprocessor-service-fabric head:master

Update AUG 2017
They changed it at VSTS, such if the colon is present it will fail auth.  The above description have been updated.