Add US counties to django-localflavor-us

I needed to use counties in a project and generated this list of county choices using the ANSI standard (INCITS 31:2009 - http://www.census.gov/geo/www/ansi/download.html) and grouped them by state/territory.  I thought it might be useful to include in the localflavor package for the US.  I don't considered it completely fleshed out as it's just the bare minimum I needed at the time, but if there's more functionality needed before inclusion I can add that. 

Let me know if this is or isn't useful, and if it is useful, what functionality should be added before submitting a follow-up pull request.