Merge pull request #243 from SlashDevin/master

Resync
Update SRPO driver; ripple effect of SPI acquire-release pattern.
Make low transmission power part of configuration (USE_LOW_POWER).
Cleaning up wireless example sketches for NRF24L01P.
Boosting command line build; cache core library and object files.

1. Cache object files and core library to obj/build-VER.
2. Add version variable; ARDUINO_VER. Default 1.5.7.
3. Update total rebuild script, tutto, to handle versions.
Add some cxa defaults.
Remove compiler/link error; duplicate definitions.
Update build log.