Printrboard w/ CoreXY kinematics

So here's the plan: 
- The [`Marlin_v1`](https://github.com/MissionSt/Marlin/tree/Marlin_v1) branch on this repo will keep an untouched copy of the [mainline `Marlin_v1`](https://github.com/ErikZalm/Marlin/tree/Marlin_v1) branch 
- The [`missionst/master`](https://github.com/MissionSt/Marlin/tree/missionst/master) branch in this repo is tailored specifically for our printer electronics, and should always be deployable.  Our current hardware target is the red Printrboard made by Geeetech:
   - [product page](http://www.geeetech.com/new-version-reprap-printerboard-reprap-electronics-sets-p-653.html)
   - [wiki page](http://www.geeetech.com/wiki/index.php/Printerboard)
   - [RepRap page for 'official' board made by Printrbot](http://reprap.org/wiki/Printrboard)

Here we add support for:
- Printrboard
    - temp sensors
    - fan pins
- CoreXY kinematics
- Non-inverting endstops, e.g. for contact switches instead of hall-effect sensors


TODO(gabe): Compare with [Chris's older changes](https://github.com/MissionSt/Marlin/compare/missionst%2Fprintrboard-corexy...missionst%2FMSMBot_Chris?expand=1) so we can figure out which ones to bring in.