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).
Would you mind spending the `case 2` and `default` block some empty lines to group steps like in `case 1`?