add level shifter for nanite i2c communication

[mariolukas]

[cemonds]

We should just provide the option to use the level shifter from watterott. http://www.watterott.com/de/Level-Shifter They are cheap, simple to handle and use an ic explicitly designed for i²c. I would suggest, we add the pads for soldering the needed 2 3-pin sockets and make the traces inbetween cuttable. The voltage supply of the low side of the level shifter can be connected to the IOREF pin of the shield, the high side to the supply voltage of the nanite. This way people can use a level shifter if needed, but don't need to, if they have all ics using the same supply voltage.