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.
You should introduce a device descriptor for srf02 devices, so you can handle more than one at the same time (which is typical for many applications...).
How about:
```c
typedev struct {
    i2c_t i2c;
    uint8_t addr;
} srf02_t;
```

Also I would opt for defining the sensors mode already in the initialization (because applications normally just use 1 mode) - so it would also be nice to define the mode in it's own type:

```c
typedef enum {
    SRF02_MODE_REAL_INCH = 0x50,
    SRF02_MODE_REAL_CM =   0x51,
    ... usw ...
}srf02_mode_t;
```

The resulting init could then look like this:

```c
int srf02_init(srf02_t *dev, i2c_t i2c, uint8_t addr, srf02_mode_t mode);
```

What do you think about this?