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).
There is a value assigned (L70 and L72). Why does it complain about these while `port_scl` and `port_sda` are ok?