drivers: added low-level I2C driver interface
cpu/stm32f1: added low-level I2C driver
board/iot-lab_M3: added I2C device definitions
board/iot-lab_M3: added features
tests: added i2c driver test
Merge pull request #1785 from haukepetersen/add_stm32f1_i2c

cpu/stm32f1: added I2C interface&implementation