Wanao i3 rear mount inductive sensor.

I now have this on my printer and almost operational. A few more firmware tweaks and I should be there. When it's done i will publish the relevant firmware tweaks and RAMPS connections. I _may_ detail doing this on the original Melzi board, but it may not be possible due to the limitations of that platform.

This is intended for an 8mm sensor: LJ18A3-8-Z
It is somewhat larger than the 4mm sensor many people use. If someone sends me the dimensions of the 4mm sensor i can add one with the c