properly initialize SP
fix add and sub instructions

ADD: properly calculate carry
SUB: apparently, borrow is set when *no* underflow occurs, not the other way around. I should've known that m(
add code for tracing
TODO: proper debug mode with stepping and everything
implement wrapping the way the Chip8 did it. That is, crappily.
make resolution a multiple of 64x32, because I can't be bothered to implement non-nearest-neighbor scaling and don't want parts of the window to stay empty
implement pausing of the emulator
add button mappings for quitting (Esc), resetting (Tab) and pausing (Space)
update readme