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
drivers: added driver for ISL29020 light sensor
board/iot-lab_M3: added ISL29020 interface defs
tests: added test for ISL29020 light sensor driver
Merge pull request #1786 from haukepetersen/add_driver_isl29020

drivers: added driver for ISL29020 light sensor