drivers/srf02: adapted ultrasonic ranger driver to general i2c interface

This PR is based on  #1859. It changes the i2c commands in the sensor-driver to the general interface definition. It also adds a small test.
For some reason you need to define the measure-mode before each measurement. But I think it's totally fine to call this function with `srf02_mode_t mode`. You are asking for a distance and just define the unit. Also you have the flexibility to change the unit without a need to call `srf02_init`. Ok who would do so, but it's still possible :-)