Line dot style

So the designer I was working with wanted square dots instead of round ones on a line graph

![square-dots](https://f.cloud.github.com/assets/52154/2112771/58ca3398-9023-11e3-8935-d8676cdaa880.png)

not sure if this is something you want to incorporate into gruff, if you do I can clean up this pull request, somethings that you might want changed?

* my editor did whitespace stripping so commit diff has a bit of noise. Is that an issue?
* new `dot_style` attribute is using a string value, would you prefer a symbol?
* naming things: renderer or drawer? it seems like draw is more prevalent in the gruff code base; currently using `module DotRenderers` would you prefer `module DotDrawers`