Optionally Specify SPIClass in beginSPI

[pschatzmann]

For I2C we can pass a TwoWire object to the beginI2C() method.
However for SPI we can't pass the SPIClass and SPI is hard coded!

There are quite some processors that have multiple SPI ports and since the SPIClass is part of the Core Arduino API, I don't see any reason for this restriction.

Therefore I extended the method to be

    bool beginSPI(uint8_t csPin, SPIClass &spiPort=SPI); //Communicate using SPI