Minimal build for Printrboard

Target Teensy-derived boards, including the [Printrboard](http://reprap.org/wiki/Printrboard), without the [Teensyduino installer](http://www.pjrc.com/teensy/teensyduino.html) and without needing the full Arduino IDE.

There are two parts here:

  1. Add `teensy` core source code files to the `ArduinoAddons` directory.  This code comes from the Teensyduino installer.

  2. Include them in the `Makefile`, see [here](https://github.com/ErikZalm/Marlin/pull/784/files#diff-04c206a8bd7b5cedd4c30700d062b19aR258)

With this, we can now build for Printrboard on Ubuntu, where the only dependencies are `make` and `arduino-core`:
```
sudo apt-get install -y make arduino-core
cd Marlin
make HARDWARE_MOTHERBOARD=81 ARDUINO_INSTALL_DIR=/usr/share/arduino
```
(Thanks to PJRC.COM for releasing this code under open-source licenses, to allow for this.)