Fixes for Pixy SPI interface with Arduino DUE

+ Added #ifdefs for DUE specific SPI.SetClockDivider
+ Added Pixy.init() to be used to init Pixy in the setup() function.
+ Refrain from callling SPI.begin in the constructor (global scope) as
  a workaround to the DUE Serial bug. [As of Arduino 1.5.6-r2]