Handle redundant FAT tables

[natevw]

Right now we're only writing to the first FAT table. There are multiple, that we should keep up to date. (Also FAT32 might need more handling for active/inactive ones?)

[natevw]

To be honest, I've been really _dis_inclined to correct this given the performance problems we're already having on Tessel. However, "double bookkeeping" here may not add quite as much overhead as initially thinking, really only an extra read+write or so when expanding/truncating files.

[natevw]

See also #19 for another reason (and presumably guide the approach) to do this.