Add an index of hexagons—hexdex ⬢

Some chapters have made some really cool hexagon logos for themselves and it would be rad to have an index of them much like the [octodex](http://octodex.github.com).

This PR adds a page `hexdex.html` with rows of hexagon logos (added to the `images` directory) that you get to from a link on `chapters.html`. 

- [ ] are there other hexagons not on here?

![screen shot 2014-11-14 at 4 29 10 pm](https://cloud.githubusercontent.com/assets/1305617/5049168/642f5220-6c1b-11e4-8947-45bf17142c92.png)