General:
- Comments, lots of comments. (Not finished yet)
- Code cleanup and refactoring;
- Misc bugfixes.

APU: Added VRC7 FM sound chip support, not enabled yet (probably it will be rewritten, anyway).

Board:
- added SXROM to boards supported for mapper 1;
- added mapper 52;
- added mapper 142;
- added mapper 151;
- added mapper 234 (Maxi 15).