Support for STM32F3 processors

[cjheath]

These processors have an I/O crossbar architecture that doesn't fit with the way stm32plus works. I tried to figure out how to do it, and even asked Andy for help, but didn't get too far. Though my C++ fu is strong, the code seemed fairly large and obtuse at the time and I couldn't figure out where to start changing things. If anyone cares to offer advice or help, I'm willing to have another go.