Driver for firing multiple i2c sonars individually

New driver for firing up to six i2c sonars to our pixhawk. Based on how much sonars are detected it choses an appropriate firing scheme to miimise interference. Simply connect the i2c sonars through i2c splitter(s) and make sure they each are assigned the correct i2c adress as noted in the driver. 

Status: Compilation is succesfull and testing has been done with the DIY/PX4Firmware fork, but not yet the master fork.
We skip 69 as it is already used for the ADXL345 accelerometer (we thought). But perhaps we can use 0x63-0x68?