Added count argument to read/write methods

The count argument in disk_read() and disk_write() is now passed to the FATFileSystem instance. This allows more efficient disk drivers to be written that can read and write multiple sectors at a time. For example, SDFileSystem could be re-written to use CMD18 and CMD25 when reading or writing multiple sectors in order to decrease command overhead.
I understand your concerns about this code, but I can't take it in, sorry, it might break existing projects. I'd rather find a better fix.