(Reduced) FAT file system support added

FAT16 and FAT32 partitions
Read and Write
Root directory only
Short file names (8.3) only
Uses a sector buffer cache

Libraries libfs.a and libfatfs.a added
(libfs.a must be linked together with libusb.a in any case)
classes CPartition and CPartitionManager added
classes CFAT, CFATInfo, CFATDirectory, CFATFileSystem, CFATCache added

sample/15-files added