o Added new Controllers type to represent controller/expansion ports on
  an NES.  Implements MappableMemory interface.

o Added NTSC/PAL_PPU_CLOCK_DIVISOR constants

o In NewNES, divide CPU/PPU clock by appropriate divisor based on region bit
  in ROM's iNES header.  Also, add PPU's memory maps.

o Update NROM.Mappings() to support new (fetch, store []uint16) return
  values.