Add support for multiple jenkins jobs and add 'TerasologyMulti'

* Add note about saved games and different types/versions
* Add changelog header

Please test and review!
Please don't merge and don't create a new jenkins build.

Support jenkins job / git branch:
* TerasologyStable / master
* Terasology / develeop
* TerasologyMulit / multiplayer

We should not change job names or associated git branches.