Tufte breaks and labels

Show the min/max values on plot axes. Looks great with ggthemes theme_tufte's short axes, as the ticks end the axes.

![tufte-rangeframe](https://f.cloud.github.com/assets/6689802/2184516/826a82ca-97c1-11e3-9878-a97d7862c366.png)


The syntax is a bit cumbersome, as the axis variable must be repeated as a parameter in the function call - but I haven't found any other way to access the variable.

Let me know whether you're interested in such extension---if so, I'll comment the code.