animated choropleths, vignettes, built in data

More details about this release can be found here: https://groups.google.com/forum/#!topic/choroplethr/gTKhxrr_rVA

1. Support for animated choropleths. Type ?choroplethr_animate to see a runnable example of animating a choropleth of all US election results ever.
2. choroplethr now ships with example datasets. Type data(choroplethr) to access them, and type ?<data set> to get more information about an individual dataset.
3. Example files now use built-in datasets, and so can run as-is.
4. choroplethr now ships with 6 vignettes. You can access them by
   • typing browseVignettes(package="choroplethr") from the command-line
   • Viewing them on the wiki: https://github.com/trulia/choroplethr/wiki
   • Viewing them on the CRAN page for choroplethr: http://cran.r-project.org/web/packages/choroplethr/index.html