Improved obstacle detection

Still not perfect, but it won't be too computationally expensive if we only use brute force collision detection for the last ~180 edges.  I don't think there's any other way to do it!  The graph now looks like this:

https://www.dropbox.com/s/7gg379dtmdrfbrq/Screen%20Shot%202013-11-16%20at%202.57.59%20PM.png