Part 3 (Req #2): Update #2 emulator

- Add bool member variables for timers
- Add ref. commentary from
http://devernay.free.fr/hacks/chip8/C8TECH10.HTM#0.1 and a few more
instructions for ST and DT to set bool when register is non-zero.  To be
used in an interrupt or to check for decrementing registers.