Nice, interrupt driven example that also tests masking and sending of frames

Code to flash the LEDs for CAN TX/RX

Added interrupt driven software buffered CAN receive and some preliminaries to support interrupt driven and software buffered TX as well.

Added code to support the masks and filters
Merge pull request #1 from collin80/master

Thanks to Collin for adding filter and mask support, 8-deep CAN Tx/Rx buffers, and a good interrupt method to call from an Arduino ISR. And augmented example code to exercise some of his additions!