Split numeric vectors of lat/lng into lists for polygons automatically

I guess the convention of using NA to separate polygons in R is fine, and this PR converts numeric vectors into lists internally.

![screenshot from 2014-12-09 15 57 38](https://cloud.githubusercontent.com/assets/163582/5366760/12776742-7fbc-11e4-90c3-77b9f56a678b.png)