census blocks added

Howdy folks,

I see a lively discussion has already ensued on hackernews ([My personal fave](http://www.flickr.com/photos/vtcraghead/9739633353/)), but I thought it'd be worth adding some geometry and variables that could better serve as building blocks of an analysis. 

This pull req includes a geojson file of SF census blocks with crime count per block, mean elevation, 2010 population and a few derivatives like population density, crime density etc. I've also put up another cartodb viz that overlays elevation and a normalized crime index.