Renamed _IOPin to Pin. Pass function to __init__ that maps index to SoC pin number.
This is fist step to introducing pin banks for on-board GPIO
PinBank abstraction for on-board GPIO.

Different pin-banks are exported from the quick2wire.gpio module:

  gpio        - GPIO pins numbered 0..8
  pi_header_1 - the pins on the Pi's header 1, except power and
                those labeled do-not-use, numbered 1..26
  pi_broadcom_soc - pins identified by Broadcom SoC pin number