Make non-existent value states readable.

See https://github.com/daguar/easy-choropleths/issues/10.

So I realize in my dumb hindsight that the issue isn't the regional values themselves, it's the interaction between fillColor and color as the individual regions are styled: basically white on white.  My proposed solution is to give a default outline of light grey, so states can always be seen:

![screen shot 2014-05-19 at 9 36 14 pm](https://cloud.githubusercontent.com/assets/461046/3022943/4ed14a02-dfd8-11e3-8cee-1752c078da25.png)

I think it also makes sense that borderColor could be a configurable value (via url params), and could include that as a separate PR. 

Since this is a bit more design-y than my prior hopes of it being a simple configuration issue, I understand if you have some reservations!  Lemme know what ya think.