Added game logic and drawing functions.

I am a few steps closer to having a working prototype.  I've added
several functions, some of which I'm using already and some which
I believe I may need down the line.  I'll take them out as I see
fit.  Right now, running main.py as-is will show a blank grid with
some block (right now, a yellow 'S' block) drawn on it.  Pressing
the R key will rotate it 90 degrees.  At this point I believe this
works with all block types.