Refactor pin reading

Refactor SPI class to general GPIO class that starts SPI and standard
pin calls. Add functionality to read several pins at same time.
If station is not selected, use GPIO

Three GPIO pins are used, number 3, 4 and 5 according to wiringPi and
15, 16, 18 according to their place on the Rasp.  Pin 5 is the MSB.
Together the three bits determine the station whose data is used.
Fix bug with station selection

Forgot to sum the bits
Add test to check byte ordering
Merge branch 'gpio-interface'