Fix writeBytes length

Credit to wilsynet
Use SPI transaction, if supported by SPI library
Use only necessary .h files in example
Merge pull request #3 from PaulStoffregen/master

SPI transactions
Unintrusive SPI transactions & misc fixes
Update README.md
Update README.md
Update README.md
Update README.md
Update README.md
Update example to work with MoteinoMEGA