Change Invalid OPcodes to a print statement + NOP

[kraln]

Some games (such as Puzznic) use invalid opcodes--there's a ton of them documented in detail here:

• http://www.ffd2.com/fridge/docs/6502-NMOS.extra.opcodes and
• http://wiki.nesdev.com/w/index.php/CPU_unofficial_opcodes#Games_using_unofficial_opcodes

Note that most of the opcodes used are NOPs, so I default to NOP + warning. Proper emulation of all of the illegal opcodes is probably warranted for full compatibility.

This fixes #26