SD card and fat16 support

[koenkooi]

I love being able to start a build on my makerbot and unplug the computer to do something else. Being able to use SD on a 328 or bigger would rock!

[triffid]

I concur.. first step is using an SD card as a simple buffer device, as I've read that the fat libraries are fairly sizeable

[koenkooi]

It probably only makes sense to support fat16 on 328 and above