Pure python, 2 or 3 compatible, remove py-spidev

- Now implemented in pure python, only standard library, no external
  packages.
- Now compatible with both Python 2 and 3.
- Probably at least or more efficient than previous version which used C
  based py-spidev because this implementation caches the spi read and
  write transfer buffers rather than build them at every transaction. So
  e.g, the read() call is just one Python ioctl() and an unpack().
- Ensure passed data for write() is limited to byte value.
- Added Makefile.