Round one of Gradle / Artifactory / Jenkins overhaul

This is a bunch of pending changes I've had for some time, plus @msteiger's Gradle 2.1 upgrade, plus some additional bits prepared today and yesterday. It starts addressing better setup with Artifactory, Jenkins, and Gradle in general.

I've tried to take the best of both worlds - the existing setup that supports using a local gradle.properties file with override values so you can test Artifactory publishing locally without Jenkins + the nicer setup from gestalt-module by @immortius 

It publishes artifacts to Artifactory again (a piece was missing) and I've confirmed you can override the target repo via setting on a job in Jenkins. This opens some interesting possibilities.

I'm going to go over it in more details in the forum as part of the logistics arc: http://forum.terasology.org/threads/logistics-arc.976/#post-10764 - welcoming feedback there in particular, this is mainly so I can show the changes. Might add a few more commits before wanting to merge this :-)

Completes #1342 

Edit: Oh, check out the template gradle.properties. Filled in with details (and placed in project root) it can be super handy :-)