SPI improvements

[konimaru]

Clock will run at 4 MHz with a sustained 3.33 Mbps average bitrate over multiple bytes.

Interested in a 6.66MHz clock and 5Mbps average bit rate?

[DavidZemon]

Of course! Thanks for opening a ticket for this! It's something I've wanted for a while but never had the sample code in front of me to implement :)

[DavidZemon]

I'm going to close this issue in favor of the method provided in #97. It looks to be very similar, but uses an unrolled loop.