Additional opcode fixes; add nestest.nes ROM to repo.

I think all official opcodes work correctly now. Here is the list of
fixes made this time around:

FIXED: Opcode 0xED (SBC, absolute mode) is missing cycle count.
FIXED: Opcode 0x36 (ROL, Zero Page + X mode) had incorrect cycle count.
FIXED: RTI needs to set FLG_NOT_USED in processor status.
FIXED: Indirect Indexed doesn't wrap to Zero Page when address is 0xFF.
FIXED: Indirect Jump address resolution not working.

Next steps: Make debugger more robust, then work on PPU (joy...)