cpu/stm32f1: added I2C interface&implementation

Here it finally is: the so far missing I2C low-level driver interface and a first implementation for the iot-lab_M3/stm32f1.

Please test, as I am not too familiar with I2C (yet).
I don't understand cppcheck here. And I am reluctant against putting a `pin_x = 0` in just for cppcheck to be happy...