Staggered timeouts, Dual head mode

- Provide staggered timeout intervals to reduce errors
- Provide staggered timeout periods to reduce errors
- Assign default txTimeout period per staggered timout period
- Add dual head (dual radio) mode
- Added overview of tuning to documentation
Raspberry Pi support

Added support for hardware SPI on RPI
- Fixed a few bugs with single SPI transfers, timing, etc
- Aligned with optimized RF24Network 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
fix typo in readme for RPi
Merge remote-tracking branch 'upstream/master'

Conflicts:
	RF24Network.cpp