GitLab

- Bring beginTransaction inline with Arduino API
- #define SPI_HAS_TRANSACTION in SPI.h
- #define the spi speed on both Arduino and RPi/Linux using
RF24_SPI_SPEED instead of different vars
- Chg beginTransaction to use bitOrder & mode instead of CSN
- Add delay directly to chipSelect function (helps ensure pin is active
before beginning SPI transactions)

#173