Attempt to fix kerning problems

See issue #2. I'm a complete novice at font things, so I don't know if I've done this the *right* way, but I think the result looks better.

Here's a before and after, 18 point, using GIMP:
![xkcd font kerning before](https://f.cloud.github.com/assets/327925/1044187/f8d126ae-100c-11e3-8c28-cbb50d8374c1.png)
![xkcd font kerning after](https://f.cloud.github.com/assets/327925/1044212/706a286e-100d-11e3-9475-ab01966bdab3.png)