Add PAL support.

 - Look for "(E)" and "PAL" to guess PAL from the filename (the iNES 1.0
   header does not provide this information).

 - Run 3.2 PPU cycles for every CPU cycles in PAL mode by running one
   PPU cycle extra every five calls to tick().

 - Add PAL versions of various APU LUTs.

 - Fix typo for the first frame counter clock for NTSC. Should be at
   3728 APU cycles, not 3278.

 - Add ROM-specific (de)initialization functions with a "_for_rom()"
   suffix, called from (un)load_rom(). Need to know whether PAL or NTSC
   to initialize some things now.

 - Turn CPU and PPU timing constants into variables and add an
   init_timing_for_rom() function.

 - Turn some performance-critical functions into templates with
   PAL/NTSC-specific parameters.