Raspberry Pi support

Added support for hardware SPI on RPI per jscrane fork:
- Fixed a few bugs with single SPI transfers
- Aligned with optimized RF24 library fork
- Updated bmc2835 files per
http://www.airspayce.com/mikem/bcm2835/index.html
- Merged bcm files with changes from
https://github.com/farconada/RF24Network
- Added example files
- Added setup info to readme