Change "bit plane" references, change endianness of compressed length bytes

The "bitplane transform" was not a technically correct name for what the
algorithm was actually doing, so it has been renamed.

Additionally, the byte order for the 12-bit data length at the beginning of
a compressed block has been reversed to put the unused 4 bits at the start
of the file. These bits may be useful in future algorithm revisions.
Greatly improved README file documentation