Added I2c board driver for firing analogue sensors

Added I2c board (pcf8575) driver for firing analogue sensors (mb1000).
This branch is based on the ArduCopter-3.2 tag of the PX4Firmware as we are using this one at the moment.

There is a problem with the driver. I am able to start the ioexpander called pcf8575, it responds with: "[PCF8575] on I2C bus 1 at 0x20", which is the address of the I2C device. But when we test it, it gives an error: I am able to start the ioexpander called pcf8575, it responds with: "[PCF8575] on I2C bus 1 at 0x20", which is the address of the I2C device.

Corresponding forum posts: https://groups.google.com/forum/#!topic/px4users/05DzKVJQu4w