Add slave mode support to SPI library [imported]

This is [Issue 965](http://code.google.com/p/arduino/issues/detail?id=965) moved from a Google Code project.
Added by 2012-06-24T21:16:49.000Z by [n...@gammon.com.au](http://code.google.com/u/111479721553138312180/).
Please review that bug for more context and additional comments, but update this bug.

Original labels: Type-Enhancement, Priority-Medium, Component-SPI
### Original description

**What change would like to see?**

The SPI class currently has a method attachInterrupt() that simplifies getting SPI transfer completes to trigger an interrupt. This does this:

void SPIClass::attachInterrupt() {
SPCR |= _BV(SPIE);
}

However there is no method to turn SPI into slave mode, which is where these interrupts would be useful. Suggest adding:

void SPIClass::setSlaveMode() {
pinMode(SS, INPUT);
SPCR |= _BV(SPE);
}

**Why?**

It seems unbalanced to have a method to do half the job of setting up an SPI slave - turning interrupts on. It would be more complete to also be able to go into slave mode.

**Would this cause any incompatibilities with previous versions? If so, how**
**can these be mitigated?**

There would be no incompatibilities because this method does not currently exist.