Ultimaker 2 clone base plate - MK2b ready

This is my adaptation of Ultimaker bed plate for MK2b heater.
I've made 2 versions:
- One is based on original design for Ultimaker 2 heater with added mounting holes for MK2b heater.
- Second version is designed for my [custom heater plate](http://www.thingiverse.com/thing:2102227). It has all mounting holes in 3,2mm diameter since I prefer m3 nylock nuts for heated bed leveling over UM2 thumb screws.

I recommend to laser cut it of 4mm PA6 aluminum.