Simple I2C example for STM32F0 family

[JiriBilek]

[karlp]

I'm not accepting any other generally cut/paste of "examples" that require external hardware. They're impossible to maintain. I'll leave it here for reference, but it's ~never going to merge like this. The only future is examples of APIs that are hardware independent, and then tiny board specific "demos" For the latter, see libopencm3-miniblink, the former is unfortunately work in progress.