Couple bug fixes; slight readability changes

@j2kun really enjoying the blog posts! I don't know a ton about Elliptic Curve algebra, so it's been fun to play along. I expanded your tests and found a couple issues (if you checkout 79449c you'll see the failures). I'm not entirely sure what should happen when comparing points on different curves, but I assume that doesn't make any sense and should raise an exception. If that's not the case, let me know what should happen and I can fix the changes I made.

I also found the `self[0]` and `self[1]` unnecessary and unreadable (particularly because you excluded your definition of `__getitem__` from your blog post version) and so I removed it in the last commit (I updated the code to make it more readable and not use the notation in the first commit). I wasn't sure if you did have a reason for wanting that syntax down the road, so I made it the last commit incase you aren't interested in it.

Keep it up!