Support for teensyduino, https://www.pjrc.com/teensy/teensyduino.html

This patch adds basic support for teensyduino.

Teensy 3.1 is an ARM-based dev board which is compatible with the Arduino IDE and can use many standard Arduino libraries unmodified. The teensy developers provide an Arduino IDE addon -- teensyduino which extends the standard Arduino IDE with support for the teensy platform.

This branch attempts to add similar support to Arduino-Makefile. This mostly consists of reading extra fields from the boards.txt that are not normally present. These fields allow teensyduino to override the standard avr-based toolchain with an arm-based toolchain.

My impression while making this patch is that Paul did an excellent job embracing the existing Arduino way of doing things. This patch should retain full backwards compatibility.

An example of using this fork can be found here:

https://github.com/stepcut/teensy-blink

I'm happy to answer questions and make modifications based on feedback.