STM32F3 initial support

Hi everyone.

Here is the cleaned up and rebased pull request of the changes @memeruiz provided to add stm32f3 support to libopencm3.

I still think some things could be improved, like for example the adc and rcc library code could probably be cleaned up a bit more, so that it uses the functionality dispatch system we have in place for stm32.

But I think it is much less invasive then it was before.

Please take a look and let me know if you still see any showstoppers that we need to address before we can merge this into master.