Minor wording edits to introduction section

I finally gave the paper a read through. I'm really impressed by what's been put together. I have a few minor wording edits, which I'll separate by section. I haven't been closely following discussions about style (everything from language formality to line wrapping), so I apologize if I've violated something that has already been agreed upon.

Note that this includes a subtle change from a double-dash to an actual, unicode em-dash. (This seems to be converted correctly from rst->latex->pdf.)