Write basic SPI bus interfacing code

I can use its techniques to get my sequencer board to talk to Nina's
UI board.

Squashed commit of the following:

commit 3d9d890ad40edc7be89f361509b43f0f9708b6f7
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Mon Apr 1 00:13:57 2013 +0100

    Let other chip keep up

commit 939a1e7b119ab6bb19245b86614f41ffe4a5a20e
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Mar 31 23:19:22 2013 +0100

    Make easier to read

commit 69c11617c54e82c7e25c8fdec1e081b382ad6a71
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Mar 31 23:13:37 2013 +0100

    Print out the returned bytes

commit 79e4ba1ebf6667fd8a1539d801a1ca533317d02c
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Mar 31 22:57:11 2013 +0100

    Implement LED cycling

    This *looks* right on the oscilloscope, but without the slave working
    yet, I can't be sure.

commit f210055d067e28965ca1fbcab1c9c05eddc07992
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Mar 31 21:56:25 2013 +0100

    Empty commit; note

    Verified with oscilloscope that the SS going low and high is working
    beautifully.  All that automated changing of its state only applies
    if you specify the pin number, which we're not, as we're not using
    a Due.  So everything's fine at this end after all, and no modifications
    to the code are needed.

commit 73883414f9279b7787109b55bc145bb0907278b9
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Mar 31 21:21:22 2013 +0100

    Ditch the Due

    It really looks like the Uno should be capable of multibyte
    transmissions if I learn how to do it right...

commit d2142fd0d16b82eef984880a0b05a669f180e42a
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Sun Mar 31 02:06:59 2013 +0100

    Make temporary SPI bus testing code

    Unfortunately, it looks like the regular Uno doesn't support multibyte
    data...