Two bug fixes; sucessfully read signature and fuse bytes from ATMega328P

- Data is shifted through the SPI interface MSb first
- The ENTER_ISP command also requires transmitting a command to
  the device. This now occurs, although the result is not checked
  as the data sheet asks. What would be the best way to implement this?

An ATMega328P, never programmed before, and wired directly to a PMOD
connector of a Digilent Nexys 3, was successfully made to enter programming
mode, return its signature and its fuse bytes using avrdude. Any command
that is implemented via CMD_SPI_MULTI is now supported.

TODO: Determine the meaning of the 7 parameters passed with the
enter ICSP mode command.